Mac 11 9mm receiver flat

Jig is machined to the exact dimensions of the inside of the receiver. It can be used with either the 9mm or the versions. It can also be used to bend the back plate in the correct location. Correctly aligning the parts and holding everything together while welding is the two hardest part of any build, this jig helps make that easier. It will work with all receivers as long as they are made to original specs and can be used multiple times. Each jig comes with all the necessary hardware to install the jig for standard SMG and semi-auto.

This cut should be made down the entire length of the piece. An alloy steel such as is best, but used with care, cold rolled mild steel will work fine. The two sides must be machined so the thickness is diminished to the basic reference size of not less than 1. One end of the core block must be machined as square as possible. It too, is a forming edge. If milled, the finish should not be rough. It costs more, but grinding to finish dimension is a much better finish.

Cobray M11 9MM SMG STYLE LOWER Receiver M 11

However, a fine finish can be achieved in a mill by a competent machinist. Locate and place the front stock mounting hole centered in the width of the forming block and 1. Location should be placed only after finishing the two sides, bottom, and one end smooth and square. The bottom corners of the width are struck with a file only hard enough to 'break' them from being sharp. Using this hole as zero move 6. Only a little stoning or flat filing is necessary after radiusing the bottom corners of the sides and tab forming end by hand. This block is finished with clearance cuts made in the tab forming end at the top as shown in drawing number 9.

This will allow clearance of the first tab bend at the bottom of the final form stage. Again, being certain to keep it on center. You now have three guide pin holes. Only the sear stud hole guide pin need be changed for forming from one flat to the other. If you are using a piece of material other than 2" wide for this block, just remember to leave one inch of the original hole size unchanged from the bottom up.

The purpose for the clearance in these holes is to make the removal of the guide pins easier, as they often stick no matter how much grease is used, and the finished frame is much more easily removed from the die by FIRST removing these guide pins. They also, don't get broken so often that way! Next, make the tab folding block to form the first fold in the extending tab of the flat, and the second bend formed in the final fold procedure.

The finish dimension of the width of the block is the formula width of the cavity cut by. It should be three inches long. Cold or hot rolled mild steel will do fine, though once again, tool steel alloy is best. Machine to dimensions and square one end. The squared end will face against the squared end of the forming core block to make both of the tab area bends.

The width need not be a slip fit, but should be held at. However, if it is held to a clean slip fit, it may be used to dimension the drilling fixture covered later. Be sure to read through to that coverage before doweling it in place. Drill and ream the mounting holes in the block as shown in drawing number 10, and transfer them into the cavity cut from one end of the plate. Be sure to use flat head screws to mount the block. Countersink chamfer the screw holes for the flat heads. The reamed holes can be either slip or press fit. The block stays there.

Be certain to relieve the sides from the front corners as shown for the travel of the flat's sides during forming, following the dimensions shown in the drawing. Failure to do this will result in certain damage to the tail sides of the frame during forming. Before proceeding, be certain to have the clean, squared, radiused end of this block facing into the remaining cavity area.

Should you make a mistake and radius the wrong side of the block, just radius the correct side and go on. It will not hurt the function of the die. Set the dowels and screws and proceed. Divide the formula cavity measurement in half and use this dimension as center line for the placement of both the sear stud hole and the frontmost stock mounting block hole guide pins. Drill and ream the front stock mounting hole first using a modified version of the formula for the cavity cut. This time use the distance from the face of the tab forming block, plus the dimension used for one side in the cavity cut formulation.

Flat thickness plus tolerance and add 1. Once again, it may be necessary to compromise the thickness allowance for forming more than one flat with varying thicknesses. Drill and ream this hole for a slip fit to the dowel pin. The forming core block is placed in the cavity cut, locating the two holes of the stock mounting hole location cavity and core together by the guide pin.

Thus held, use two pieces of metal shims or, drill bits the size of the metal thickness plus the tolerance space at the front, one on each side to center the block in the cavity cut. This is the best part, because it is the last part of the die! It is the gage block for forming the first bend in the extending tail tab. The drawing number 5 details this block. If you have chosen a piece of metal wider than four inches for the cavity half of the die, you may wish to cut a piece as long as the width you have chosen.

Room for variances has purposely been left in this, as this is a 'survival level' construction report, and the reader may not have available anything more than scrap to chose from. Assuming the cavity is four inches wide, the finished, squared length of the gage block is also four inches. Just as the cavity cut was made as closely to center as possible, so will the clearance cut in the bottom of the gage block be made so close to center. The clearance cut however, will only be. The clearance cut may be. However, for the commercially available flats, the clearance cut need only be flat thickness plus.

Drill and ream the dowel holes and drill the clearance holes at the dimensions shown. It is not necessary to counterbore the screws. To locate the gage block, use the prepared flat, either factory or homemade, and place it centered on the cavity plate REVERSED to the direction of the final form position, so that the tab hangs over into the cavity forming area.

At the same time, the length of tab protruding into the cavity forming area is measured and the flat is kept centered on the cavity plate and squared to it. Carefully clamp the flat in place solidly, and place the gage block over the tab, and firmly against the side 'wings' sticking out to either side. These 'wings' will be the gaging surface to form the first bend of the tab. This is why the builder was urged to recognize the importance of squareness in the section covering the manufacture of the flats.

Thus located, clamp the gage block in place and transfer the screw holes. Drill and tap them into the cavity plate. Then return the gage block to it's position and lightly tighten it down to the cavity plate. Insert the flat again, remeasure the tab distance and double check the square of the gage block. Tighten the mounting screws, double check square after tightening, and drill and ream the dowel holes.

The best doweling fit for these holes is to ream the cavity side of the holes as a press or even a tap fit. The holes in the cavity side may be blind drilled, not coming through the plate completely. It is best that the dowels only protrude the height of the gage block from the cavity plate. The dowel holes through the gage block should be a slip fit, or even a carefully drilled hole, using a lot of lube, and pecking as you go til you pop through the other side. This plate will have to be removed after each flat formed, and left off for the second and final form stage of the receiver channel.

Location of the flat for final form will be determined by the guide pins. How well it forms is determined largely by the radius hand cut into the top guiding edges of the cavity cut on both sides, and the radius on the tab forming block at the rear. The radii on the matching surfaces of the forming core should be very small, actually just breaking the corners.

The better all forming surfaces are polished, the better the die will work. In both stages of forming, the liberal use of lubricating grease is advised. Lube everything well, including the guide pins and both sides of the flats to be formed at least in the central areas, and the cavity of the die. Too much grease is not a problem here.

This is especially true of soft mild steel. Since the die was not designed or built for massive production it surely doesn't have to last a long time, but the builder will want a finished product to be proud of; not a formed flat that has scrapes and gouges from galled metal and pry bar marks getting it loose. The grease will come off. The marks in the steel are another thing. There is still work ahead, so why make more than necessary.

Then, when placing it in the final 'channel' form position over the guide dowels, turn it so that the first bend just made is pointing up and the markings are down. Location of the right side relief is critical that the channel finish being formed with it to the right. It may seem stupid, but it is an easy mistake to make that only creates more work to clean up. It may be an added precaution to use a black magic marker and mark the flat on the side that will be the outside bottom when it is finished.

This may be especially helpful with the homemade flats that will have no other markings. When loading the flat for first tab forming, be certain the mark is up. For the second and final channel form, be certain the mark is not visible. If the homemade flat is formed without the relief, it will reduce the possibility of an error.

It is not, after all, much work to hand cut the relief into the right side. Perhaps it seems that the forming die is a lot of trouble to make to produce a receiver channel. That may be, but compared to every other option explained, and a couple avoided like the plague,. In the wildest of oppressed conditions, "pretty" really isn't a consideration. But, in the study of mechanical engineering, very much like the study of welding, well structured metal formations are more than just pretty.

They are stronger. Also, bear in mind that the frame still has to have precision holes placed, and that is best done from a flat square premise, not a bumpy, lumpy, jagged, crude formation. For the perfectionist, the job can't be done without this die! But regardless how you have formed the frame, it is important at this. The square of the back of the sides to the length should have been well checked before forming. Let's drill some holes.

The largest obstacle in the way is a bent piece of metal inside the frame to the front. It should be obvious that one of the tricks to welding sheet metal and avoiding warp, is to fill holes, thus minimizing metal shrink. Therefore, careful examination will show that the bottom center weld in this bent metal piece can be seen pretty clearly in the metal finish. It is centered in both directions inside the two bends of the piece.

Actual location from the front of the gun is 1. Only the sear stud hole has been eliminated. To avoid any more holes than necessary, this weld will have to be cut carefully to remove only the metal from the bent piece, not cutting through the bottom piece. Of course, the simplest method is to drill a hole through both pieces to remove the weld, but then you have a hole to fill or leave for a drain.

The sear stud hole still must be placed at one inch centered. So, for the least hassle, drill the sear stud hole on location through both layers and cut the weld in the center of the bent piece only through the thickness of that piece. Do this after removing the entire trigger group parts set. Then, after removing the bent tab weld to the right side, a sharp tap with a cold chisel at the lap of the two in the front should knock out the bent piece, however, the best method is to mill out the area surrounding the weld and forward to the inside front edge of the front bend, using care only to cut the metal of the insert piece and not the frame.

However it is accomplished, care must be taken not to change the square shape of the frame. Be certain to clean up any rough weld left after the cutting or drilling is done. With the safety switch removed, there are now two holes. These safety holes are useless, as the SMG trigger group incorporates a completely different safety design that uses the slot near the tigger. These can be welded up or filled with an epoxy filler like J. Weld, or the similar grey colored Loctite epoxy, then sanded smoothe and painted, or not - to your liking.

The trigger pin holes are close enough to work as they are for the SMG trigger parts set. On the bottom right side at the trigger slot there is another smaller slot in line with a hole and a detent relief. This is the mounting hole for the SMG safety. In the unlikely event that the production semi model gun does not have this slot and detents, they will have to be drilled and cut if the safety is going to be installed.


It is usually ignored. As the forming die is to the making of the frame, so is the drilling jig to the making of the trigger group holes. In this design, the sear stud hole is used to align the frame to the rest of the trigger group holes. It is really only time saving if you have several frames to drill.

It is again, the perfectionist's tool. Mail order tool sales companies are a part of the source lists found in the back of the book. Practically all of them stock O-1 precision ground flat stock. O-1 is an oil hardening tool steel that will not wear after heating and quenching in oil, which hardens it and this explains the 'O' in O Most machine shops will have scraps of O-1 they will most likely part with for a small part of what you can expect to pay a tool supplier. Of course, then there won't be that excellent piece of scrap left over to make a hunting knife. Other types of tool steels can be used sucessfully, too.

A2 is an air hardening tool steel, and W2 hardens by quenching in water. D2 is a tough die steel that is intended to be hardened in a controlled air environment, but will oil harden as well. Care must be used if this is done however, as it becomes brittle enough to. Three pieces, four inches long are necessary. The flat thickness used to determine the distance from outside bottom of the frame to the correct hole placement height was. No tolerance is necessary. If the tab forming block made for the forming die is held to exacting 'slip fit' dimensions, it can be used as a gage to set the inside distance between the sides of the jig for their attachment to the bottom piece.

As is visible in the drilling jig print, the two sides set on the bottom plate, flush to the bottom of the frame. If the frame is formed cleanly enough, the sear stud hole can be transferred from the bottom of it to the bottom piece of the jig. Or, be trusting, and use the blueprint. It works. Begin by marking one of the one inch wide pieces in the upper right corner, 'right', and the other in the upper left corner 'left'. Or, R and L These will be the outside surfaces of those pieces.

From this point remember that the dimensions for the right side plate are given with the zero at the bottom right. The dimensions for the left plate are given from the bottom left. These dimensions are the same, just headed in opposite directions. The height dimensions are established using the nominal flat thickness of. So long as the flats are within. If the thickness is more than that, you will need to add the difference.

Only the hole sizes will vary. See the drawing set numbered 7 through 9. Drilling them with a clean sharp drill should be enough. It isn't necessary to ream the holes to size. In fact, after hardening, it could cause some problems. A drill just about always makes a slightly larger hole. Old motor oil from. All pieces should then be polished to a silver finish It needn't be a bright finish and slowly reheated preferably in a piece of 4" wide channel iron, playing the flame on the channel instead of the O1 metal, until it takes on a little tan light straw, in the heat treat business.

Dark brown or blue is a little too much, but if these colors appear only at the corners or as streaks through the O1, it will still be hard enough to be servicable. At this point, leave it alone to cool as it will.


It should leave a light tan with brown streaks through it. With some care, this process can be done on the kitchen gas stove. It may be possible on an electric range. Tell the cook to lighten up; if the metal has been cleaned in a solvent like MEK or acetone, it won't stink, smoke, or hurt anything! Should any holes shrink enough in hardening to seize on a drill, polish the hole open with emery cloth or light circular strokes with a small diameter grinding stone in a Mototool or die grinder, fitting the holes to the corresponding pins.

The absolute, best way to finish this assembly is on a precision grinder, lightly cleaning-up the top of the bottom plate, then, in assembly, grind the insides parallel to each other careful to stay close to the outside dimension of the frame.

Frame Weld Parts | Firearm Parts & Accessories - Gun Parts & Accessories

Experience shows that 1. To use the drilling jig, simply place the frame in the jig, front to front of jig and locate it with the screw through the tapped sear stud hole from the bottom, and use the forming core or mandrel inside the frame securing it with a clamp to the jig during drilling.

The steel used for the MAC flats is an alloy It is a bit more resistant to machining drilling and must be drilled with clean sharp drills and a quality cutting fluid. Don't rush this drilling and the holes will be clean, close, and dimensionally stable, assuring proper function of the gun.

Be patient and deliberate throughout construction, and be confident that these instructions will work if you take the time to do it right. To guarantee the best certainty of hole placement, and minimize flex in the sides during drilling, after placing the forming core of the forming die in the formed flat channel, or using the forming. Clamp from the top of the forming core to the bottom of the drilling fixture, and locate the sear stud hole through the frame and into the core with the 10 screw as described above.

Drill through the frame on each location penetrating the core block lightly. In the event that more forming is done later, the holes will not interfere at all, nor weaken the core block. Those who do not need the shop cluttered with unneccessary fixtures should procede to the side layout template and transfer the dimensions directly onto the blued up metal with a scriber tool.

While the dimensions there are drafted as closely as possible, the print was not intended to be scaled. Builders are advised to transfer the dimensions with vernier calipers or on a surface plate using 'jo-blocks'. The depth of the rule in a combination square can also be set with precision and scribe directly from the end of it as it is moved along each side, gaging from the bottom of the frame. This same process can be used from the square filed front end to set the length dimensions of the trigger group holes. Great care must be taken to use a sharp 'prick' punch to set the hole locations.

If necessary, use an optical visor, loupe, or magnifying glass to get the punch on location. The holes are then overstamped with a center punch ground to a wider angle. The dimensions to the hole centers is identical on each side, only the hole diameters varying. The best way to place the holes in the sides of the frame without the drilling jig is in a milling machine.

Notice that front is to the. Zero on both pieces is located at front bottom. Measurements are 'up' and 'back'. Or, height and length Pay special interest to the pin sizes, especially if experienced in making the M or M some time ago. The rear stock block retaining pin used to be a cold rolled pin inserted with some difficulty. Or, a 'C' clip in the SAP models that had the "semi-auto carraige" captivating thesear pin. Things have changed since Gen. Mitch Warbell ruled MAC. This new pin is an improvement over the old pin, and inserts from the right side. The trigger pin stops on the outside right wall.

It now has a low profile rounded head like the stock block retainer pin and it still locks with the clever locking spring wire. Chances are, the guys working on the guns got tired of fighting the trigger spring just to get the locking wire in place. It is common to forget to start the locking wire which must be in place under the trigger pin to lock it before inserting the trigger pin.

It was always remembered just after the fight to get the trigger spring and pin in place. So everything had to come out and be started again. See what you missed if your kit has the new pin? The new pin is also used in the semi-auto model, but is inserted from the left side in that gun. Some of the pins supplied are still the old design, so be familiar with the difference, as a different size hole in the right side will be necessary for the old pin.

These differences are noted in the notes to the side layout templates. It is apparent that the old style pins were either manufactured for use in either the SMG or the semi, or modified for use in the semi, as they have a locking wire relief ring on both ends. When ordering a kit, some difficulty can be relieved by specifying that the 'new trigger pin' is desired in your kit.

Unless the builder prefers the old style pin, which, having no head, is flush to the sides of a properly formed frame. It is supposed to be. This can be easily resolved. Take care to get it cleaned up all the way to the inside of the selector lever boss, which rides against the outside left surface, and keep the file parallel to the pin's surface. Another solution is to use a letter 'L' drill, which is. Any and all are commonly available at any professional tool supply house, or through one of the tool suppliers listed in the back of this book.

Just don't be in a hurry. And, remember why machinists love Mototool! More about fitting and tuning later. If the gun to be assembled is to be a semi-auto only gun, the holes dirilled in the formed flat will differ from the SMG, selectfire models. It is also necessary to place the hammer pin hole to the rear as shown on that drawing. Since the semi-auto unit cannot use the stock assembly, the rearmost holes can be disregarded. The same regard for the 'old and new' types of trigger pins applies to the semi and SMG models.

If the flats being used are handmade, it will be necessary to put in the stock latch pin holes at the rear, if the stock is to be installed. Using '" as the nominal flat thickness, the height is. The height of the hole is the critical dimension. This pin is nothing more than a stop for the stock latch assembly.

It's proper dimension is. Deburr all holes drilled inside and out with a flat file or the Mototool and check the pins for fit and alignment of pins from hole to hole. Minor dressing of the front upper receiver connector pin holes will be done after the 'front tabs' are welded in place. It is really a waste of time until then. Misalignment is pretty common, but is seldom excessive. The differences are obvious. This position is critical to the proper function of the gun. It is also the only area in the entire relief in the bottom for the magazine, trigger guard and trigger clearance that is critical.

The face of the relief that gages the forward position of the trigger guard is the most important. If the frame has been properly formed, and the inside dimension held close, the width of the outward shoulders on the trigger guard where it is formed for the chambering ramp will effectively hold it on center to it's proper position. The front face of the relief must be carefully measured, scribed and filed in place. If the face is a little wide, it is of no consequence, so long as it is kept centered, but it is still best held very close. Distances between the edge of the relief and the outsides of the frame should be frequently checked during the filing to keep them identical.

Do not cut forward of the scribe line gaging that position of the guard. If the reader buys a bent center section, as is available from suppliers, and the rear is cut off just behind the rearmost stock block mounting hole, this can be used as a template to lay out the bottom of the frame.

Care must still be used however, in cutting in the triggerguard relief. First, the entire relief areas are roughed in leaving them a little small in all directions. The trigger clearance slot can be cut to size, but left a little short. It doesn't have to be square, and can be left round if the roughing work is done on a mill. In this case, take it to size. A little over doesn't hurt. The magazine well is best left undersized and cut to size after the magazine housing is welded in place. Just make enough room to work in it. Some prefer to precisely cut it to shape and size and use a metal magazine to align the magazine housing for welding.

Experience has not proved the value of this procedure. A simple jig for welding will be explained in chapter four. Assembly will detailed in chapter 5, with a few touch-ups to improve function. It is arranged in 41 steps: Make flat 2. Drill locator holes: Form the frame channel 4. Deburr and correct rough areas 5. Drill trigger group holes 6. Weld front tabs inside frame 7. Weld sear stud in place 8. Weld sight plate in place 9.

Grind weld areas clean Layout bottom area for mag well, trigger guard, and trigger relief Cut out bottom relief File in trigger guard to fit Assemble upper receiver to lower receiver with pin for primary fit Check trigger guard to barrel alignment before welding Tack weld trigger guard at frame inside bottom after removal of upper Weld trigger guard at front outside Weld trigger guard inside frame behind side tabs near chambering ramp Clamp magazine housing to frame. Check square and alignment to center line of frame. Weld behind mag well centered in mag well per sketch no.

Check clearance in mag well for passage of magazine. Grind clear as necessary Assemble upper receiver to lower with pin, and adjust upper rear if necessary before welding Weld upper rear on each side top Grind upper rear welds clean Clean up weld on front of trigger guard File off any remaining burrs, and cut in the wire relief at inside left top of trigger guard, tight against the inside wall.

Prepare for metal finishing sand paper, sand blast, etc.

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Finsh metal Polish top of disconnector and bottom of sear catch areas Assemble trigger group and mag catch grip assembly Drill barrel retainer pin hole with barrel installed in upper receiver and install retainer pin Assemble upper receiver ass'y and install in lower receiver Check for function flaws in both modes of firing and correct problems Test fire Fear no evil! It must be done in a particular order to avoid complications. For example, if the trigger guard and magazine well are welded into place before the trigger group holes are drilled, it is more of a hassle to hold the frame, lay out the holes, and the drilling fixture cannot be used.

Most importantly however, is to remember not to weld the upper rear top welds before checking the fit of the upper receiver assembly with complete bolt assembly to the lower with the trigger guard and magazine housing welded in. The stock assembly is relatively simple: It would be prudent to use the stock rail section in the frame, with the block in place prior to welding to be certain that the block will be centered in the frame.

In the event that the rails bind, check first to see if there is free movement of the rail section of the stock without the block in place. In this case, also use the rail section to position the block during welding. It is held for welding by clamping or wedging, and after cleaning up the welds, the latch is assembled and the retainer pin placed and locked. The stock assembly is inserted in the frame and checked for free movement, and both rail holes and stock rails are ground or filed for necessary clearance.

So, there is a sequencial order that must be followed. No doubt, the over anxious will get the opportunity to re-read and re-do a lot of the preceeding instructions! Experience is what you get when you don't get what you wanted. Everyone can get what they want the first time out if they will exercise patience and follow instructions. The components of the basic lower reciever are as follows: The formed frame The front tabs The sear stud The trigger guard The magazine well After the trigger group holes are drilled and deburred, the front tabs are placed inside the frame taking care to keep the upward notches to the rear.

The original connector pin is not advised to be used because the heat of the weld may damage the spring under the locking ball in the pin. The factory method is to weld the top area of the bends of the tabs. Often, one side of the tabs will appear to angle away from the frame side. In this case, weld just the one laying flat to the side, and then with the alignment pin still in place, use a pair of vise grips to squeeze the errant side to the frame, and thus held, weld that side also.

If welding at the 'U' formed front edge, weld it all as one continuous weld. The thinness of the metal is difficult to weld with stick arc welders, even for the advanced welder. Therefore, the egde welding is only advised to the users of heliarc. If the builder is unfamiliar with sheet metal welding, a little practice on some scrap is advised prior to the actual welding of the frame. The simplest and quickest attachment is to weld the top of the tab sides only. It is not necessary to weld up the area along the front face of the frame to close the gap between the front tabs and the frame, but it makes a much nicer appearance, welded and ground clean.

The sear stud is next and it is best clamped in place with a pair of vise grips taking care to keep the front of the jaw on the outside bottom clear of the stud sticking out of the hole. Of course, once tacked, the vise grips can be removed to finish the weld. Just be certain that it is flat and square to the clean deburred hole before welding. Advise is hereby given to the wary: Occasionally, a Model 10 part may appear in the trigger group parts.

All of these are obvious but. Please note that the thickness of the base, that distances the contact surface of the sear to the bolt, is thicker by. If used as is, this could prevent engagement of the sear with the bolt, resulting in a "runaway gun" phenomenon like that described earlier with the fixed full auto firing pin in the semi-auto gun. Either chuck it up in a lathe or drill press and turn it to the proper dimensions, return it for an exchange, or trade it off at the next gun show for what you need. Now, grind the areas inside the front tabs clean for fit to the upper receiver, and the sear stud weld clean for appearance.

Place the rear sight plate also called the "backing plate" in position over the area of the rear tab containing the two holes for the two pins of the bolt assembly and clamp in position with a small block of metal or wood placed inside the frame. Mar 21, 5. I have one in semi. SilentHitz , Mar 21, Mar 21, 6. Gordan Ingram came up with the MAC series guns. Here is a linkie to help. It's a very twisted and overlapping array of manufacturers and designs that is confusing at best.

Mar 21, 7. Cool, make sure you post pics of your progress, would like to see your progress. Mar 21, 8. Mar 21, 9. Yes, that one is an M11A1: Very interesting stuff. If you like these guns, I would recommend the book "The Mac Man.