First let me say I am no engineer/gunsmith/or anyone who knows anything. So if you chose to do what I've done it's on your head if you try this and screw it up and get hurt.
I have built several of these. The one with the most rounds through it (300+) looks great, no wear or anything funny and only a few failures to feeds that were only happening with one particular mag. It takes me about 5-6 hours for a folder and 5 hrs for a fixed stock build.
I had several years of experience about 20 years ago welding in muffler shops and none since then. I have used both 110v home/hobby mig welders and a heavier farm type 220v wire welder for these builds. Both had shielding gas. If you are good with your 110v go for it but the 220v was the heat for me. You could TIG these too but I have absolutely no knowledge of how to do that.
Start with one milled kit:
Make sure the torch did not get anywhere near the locking lugs of the front trunnion.
Add 2 repair plates:
And get this:
Cut, file, or grind the front trunnion so the slag is removed and the cut is nearly vertical:
I used a chop saw with metal cutting blade but a dremel, hacksaw or grinder can do the trick. Try to leave as much of the lightening cuts intact because these are difficult to repair (not impossible though)
Demill the folding stock or fixed stock, removing everything that is removable:
Shear these 2 rivets from the inside.
Trim the rear trunnion like you did the front.
I prefer to leave AT LEAST the end of the upper receiver rails intact. If you have a folder you must leave the holes for the folder intact.
Make sure you take the sling loop out if it is a fixed stock.
Now mic out the top and bottom of the front stub and the top and bottom of the rear trunnion. You will probably find the top of the rear trunnion is out of spec because of the torch cut. Don't worry yet.
Build a jig for the welding. All I use is a 3/4 thick chunk of steel that I have run my file across to clean and level. Take a 3/8" 12" long piece of key stock and weld it to the edge of the plate Weld ONLY the outside edge. Clamp before and while welding and cooling. When you are done it should look like this:
It is critical that this plate be flat and the intersection of the keystock and the plate square, flat and clean of burrs and slag.
With a dremel angle grinder or file bevel the INSIDE of the right front trunnion slightly.
Place the right side of the gun onto the jig.
note that the keystock should just barely NOT contact the barrel and the top edge of the front trunnion must be tight against the keystock.
Clamp in place but do not apply so much force that you slightly crush the trunnion.
Take your repair plate, bevel the front inside edge with a file dremel or angle grinder like this:
If your repair plate has a recess pre cut for the selector/safety make sure it is the correct distance from the front of the receiver. (trim the front edge of the repair plate to achieve this)
Make sure the repair plate is tight against the keystock and clamp.
Tack weld the front trunnion to the repair piece near the top and bottom. Make sure everything is tight on the plate and against the key stock. If not fix it.
Run a continuous bead top to bottom or bottom to top as you prefer.
Grind or file flat.
Be sure to clean this area now because it will never be easier to get to.
Get your stamped rails out (or your homemade ones try aircraftspruce. com for good prices on 4130 in small quantities) Trim the bottom of the rail so there is a slight gap between it and the notch in the repair plate. I let the rail run long, and not cut it to length.
The top edge of the rail should be parallel to the upper rail and should be the exact same distance from the lower edge of the upper rail as the trunnion dimensions.
Clamp weld and grind smooth:
Take your receiver cover and "install" it on the front receiver stub and lay the rear trunnion out on the jig. This is how I determine where to cut the repair piece to get my total length. Mark your jig for the front edge of the rear trunnion and mark the repair piece. Remove the receiver from the jig.
The right side outside weld will look something like this and that is OK:
Cut the rear of the repair piece to length and bevel just as you did the front. You will note the rear stub is around .065" thick and your repair piece is about .100" to .120" thick. Keep this in mind as you bevel.
Place back on the jig and clamp. Place the rear trunnion on the jig and clamp CAREFULLY you can easily crush the top front corner of the rear trunnion. Ensure that all parts are flat against the plate and tight against the keystock. Double check for correct length by placing the cover on the set up. I prefer to have it *slightly* too tight and file a little of the cover (1/32" or so) to achieve a tight rattle free fit.
Tack weld top and bottom. Check again to make sure everything looks right then weld.
Grind or file smooth.
If your repair piece does not have a cut down area for the safety I would grind it now. Locate the correct area and take it down to about .100" or so. (.070" if you are bold) If you have a mill then do it now with and end mill or later with a key cutter.
Remove from jig
Take a straight edge and check the right side for flatness. It should be straight.
Measure the outside of the front trunnion and the outside of the rear trunnion near the weld. They need to be the same. If they are off take a nut and bolt put them inside and spread the rear trunnion to the correct dimension, leave the bolt in place for now.
Test fit the receiver cover now. If the receiver is too long cut through the rear weld and shorten. If it is too short cut the weld and reweld as well. If it fits well or is *slightly* too tight you're good.
Measure and cut the left repair plate to fit. I like to have a snug fit but not one too long where it bows the right side even slightly. Bevel the inside AND the outside of this plate. Next take your left stamped rail and cut it to length so that the ejector will be 2.300" from the rear of the barrel. Trim as needed, place on the left plate and weld it as you did on the right side then grind/file smooth.
Clamp the receiver on the jig left side down with the top tight against the keystock.
Clamp the left side plate making sure it is tight against the keystock.
Measure the mag well opening, it should be 1.022" or a little more.
Spot weld top and bottom front and rear trunnion from the inside. Remove from jig and check to make sure everything looks right and the cover is correct or slightly tight. Just look it over good to see if it looks straight. Return to jig and weld the inside as best you can (tight quarters here).
While it is clamped, cut a notch about .065" deep with a cut off wheel on an air grinder or on your dremel along the seams of the right side outside welds. Kinda like this drawing:
then weld these from top to bottom. Grind close to smooth.
Flip the receiver over, notch these welds on the left side as you did on the right and weld and grind nearly flat. It should look like this:
Clamp it in a padded vise by the barrel.
Grab a straight edge and a sharpie marker. Line up the straight edge on the bottom and mark the outsides of the repair plates to tell you where to cut the excess off the bottom. Using a lot of Dremel disks or a cutoff wheel in a die grinder or even an angle grinder remove the extra material but leave about 1/8" extra below the line.
There seems to be 2 ways of attaching the trigger guard. One way has a little milled hump on the bottom edge of the receiver and the other is like the stamped guns in that an extra piece of metal is sandwiched between the trigger guard and the receiver. Look carefully at what you have. (One of my kits should have had the extra piece but it was missing so be careful) This will determine how you cut the rest of the lower excess.
Turn the receiver one side or the other up and FILE (not grind) smooth. Use a good 14" long clean file and evenly stroke it. Clean the file often and try not to file off those factory stampings. Those of you who were demilled through the lightning cuts fear not you will fix it later. When you are tired of filing and are tempted to get it "just a little closer" with that grinder- DON'T. This is where patience really pays off. It should look a little like this:
Now I grab my bolt carrier and measure the width of the carrier across the rails. This is usually about .990" to .975” Measure across the upper rails on the receiver. I will wind up filing some off or if you’re handy with a dremel use the reinforced cut off wheels and trim both rails EQUALLY to get close to the carrier dimension. Stop short of this dimension by about .005 to .008 or so and hand file to the correct measurement +.003 to .005". Now your bolt carrier probably still won't fit because the rail is likely to be too thick (depending on how worn your carrier is). Take that trusty file and file the top of the welds and repair section evenly and carefully. You may have to spend a few minutes working with this to get it in there. Don't make it loose because the parts will wear together and remove that last little hang up. Put the carrier in and OIL it wd-40 or similar and work that carrier back and forth. If you’re still having problems solvent clean and use cheap black spray paint on the rails and carrier, then work back and forth, remove the carrier and you will see shiny spots where you are tight.
Put the carrier and bolt together. Act like you're putting the assembly in the receiver. Hopefully it WON'T fit. You need to mark the rails where the bolt needs extra clearance to fit into the receiver.
Using your dremel with a grinding wheel or a narrow hand file cut this relief back until the bolt BARELY fits past the upper rails.
Now separate the bolt and carrier. Drop the bolt onto the lower rails and slide back and forth as if it was operating. You will find the rails are too narrow. Trim those back like you did the top rails and trim back the ejector till the bolt works. Clean any burrs or snags it encounters. You may find where the stamped rails and the original trunnion meet there is a bump. Use the dremel to smooth this area.
Once I did have a problem with a bolt dragging the inside vertical areas between the upper and lower rails. Now this is a MAJOR PITA. Unless you have a milling machine. If you have this problem you have probably made a mistake when you welded your stuff together or the plates were slightly too thick. Take your dremel and a grinding wheel no more than 1/8" thick and work this down. Don't touch the rails.
Now reassemble the bolt to the carrier and try it as an assembly in the receiver. (Use a little oil for now) It should work well. It may be a little gritty but that will go away soon enough.
This next part is where it gets a little tricky. It's time to locate and drill the FCG pivots. I locate mine off the rear of the barrel and or the rear of the closed bolt.
Measure your pins, measure both ends diameters and then the distance under the head for the retaining clips. You should find that the large end is .2735" and the small end is .1955". These correspond to drill bits #9 or #10 and "I". For those of you not familiar with lettered and numbered drill bits they are available through MSC and mcmaster.com and similar supply houses. Once you have located and center punched the pivots clamp the receiver into a drill press that you KNOW is square (table to spindle axis) left side up and drill a small pilot hole 1/8" should do it, all the way through the receiver (both sides). Verify the hole on both sides is correct. Reclamp and slowly drill the receiver in steps up to the correct hole size for the RIGHT side (small pivot) Now continue drilling the left side ONLY to the correct pivot size. If your pivot s are not .120"from under the head to retaining clip you need to countersink the holes. Like this:
This also gives a nice smooth fit to the pins as the heads are nearly flush when installed
You will find you have to remove a little of the width of the hammer pivot. Just file or grind a little of both sides at a time equally till it slips into the receiver.
Then remove a little more off the left side to make room for the retaining clips on the pivot pin. Don't worry about removing this material as the hammer spring will have plenty of space to slide in.
You may find your Tapco hammer is slightly too wide where it catches on the right side on the right side opposite the diconnector. It may hit the right upper rail and it might not. Just ease the rail back with a dremel if it does or if you have a single hook trigger this piece on the hammer is useless and grind it back.
The first sons enterprise trigger group I have did not have this problem.
Now test the function of the bolt carrier/bolt and the FCG things should work well.
It's time for the safety. Drill and cut this hole where indicated. You may or may not have relieved the material in this area on the right side in the receiver. Some safeties have .100"+ where it fits through the wall that can be removed and some do not. If you do have enough it is easier to remove it as shown from the safety than cutting the receiver.
Fit the safety and make sure it engages the trigger preventing it from letting the hammer go.
If everything has gone well its time to close of the bottom of the receiver. Grab a 1" wide piece of 18 gauge steel (farmsupply shops, Lowes, Home Depot, metal shops etc.) or 4130 if you can find it. It needs to be roughly 3.75" inches long. Grab a magazine and put it in the gun how it will sit. Mark the back edge of the magwell with a sharpie. This will tell you exactly how long to cut the floor.
It should fit something like this:
Grab an extra piece of 12" keystock and clamp the floor plate to it and lay it in the receiver. Level it and spot weld the corners and 1/2 way down the sides. When you are confident it is right notch the joints like you did on the outside side welds and weld.
Now if your trigger guard has the extra pieces sandwiched between it and the floor grind and file your welds till it’s all flush. If you need the raised hump in the receiver, shape this into the bottom (the picture above has this feature)
Locate the trigger cut out off of the trigger pivot (not off plans) Cut this for single or double hook triggers as needed. You will notice there is a larger hole usually about half cut out. This is a relief hole for the grip screw. Drill it out.
I hope this will help those of you scratching their heads trying to figure out a way to do this without buying a milled receiver. Plus you can keep those original markings!
I should have m70 plates ready to ship by early February. Email me if interested ($55) a set at firstname.lastname@example.org I need 20 sets ordered for the machinist to make them.