Section 5: Completing the receiver
In this section you will install the trunnions and the trigger guard/mag catch assembly. You will also be fitting the top rails to the bolt carrier and installing the lower rails. Please review this section to determine what tools you will need. The tool and materials lists are for a rivet build and cost wise is about the same as a screw build. All of the tools you will make here can be bought ready made for around $235.00 plus shipping. The actual materials you will be using should cost less than $35.00.
You will need the following tools: You will need the following materials:
3 lb hammer 1 piece of scrap steel .040" thick x .250" wide x 8" long
Vise grip pliers 1 piece of steel .250" thick x 1.500" wide x 8" long
Assorted hand files, flat, round and square 1 pieces of steel .250" thick x 1" wide x 6" long
Drill press 1 piece of steel round .500" diameter x 4" long
Dremel with grinding bits and cutoff wheels 1 rivet .155" or 4mm diameter
Center punch Set of 24" bolt cutters
Jig saw with metal cutting blades 2 8-32 .500" long screws w/nuts
2 12" bar clamps
Bench or angle grinder
Drill bits: .156" 5/16" .375"
Soldering iron
4" "C" clamp
You will start by making several tools that will be needed to do the build. The first one is the front trunnion rivet tool. This is made by modifying the bolt cutters. You can buy these at most large home stores like Home Depot and Menards or through Harbor Freight for around $10.00. This will usually be less expensive than buying several taps, bits and a tap handle. You also won't need to deal with broken taps. You will shape the jaws to look like this:
The lower jaw is ground flat while the upper jaw is shaped to fit into the front trunnion and crush the rivet. Work slowly on the upper jaw to avoid over heating. If you get it too hot the metal will become brittle and brake during use. It will take a little test fitting to get it just right. You will also need to cut a piece of the 1" wide x .250" thick steel 1.500" long and drill a 5/16" detent in it. Use the Dremel with a grinding bit to shape it so a rivet head will sit almost flush in it. This will be the bucking bar for the front rivets.
The next tool is the front trunnion rivet locater. If your receiver already has the trunnion rivet holes drilled skip this tool. Using the .040" scrap fold it into the shape shown below. Once folded drill a .156" hole as shown. You will want to shorten the rivet shank to .250" and solder it to the hole in the short arm.
Once the front trunnion is in the 80% receiver the rivet holes will be covered. This little gem will let you accurately place the front rivet holes without a lot of hassle or "Egging" of the holes to get the to align. When you insert the short arm into the trunnion position the rivet in the hole and the outside hole will be perfectly lined up.
Next up is the trigger guard rivet jig and rivet punch. This jig is used as the bucking bar for riveting and to position the trigger guard on the receiver. Start with the .25" x 1.500" x 6" piece of steel. Mark a center line length wise and drill a .375" hole 1.750" in from each end. Using the jig saw with the metal cutting blade make two cuts length wise from hole to hole. You will now have a slot 3" long and approximately .375" wide.
As you can see the trigger guard does not just slip in. You will now need to finish the slot with files or the Dremel. Square the ends and make the slot only as wide as is needed for a tight fit. The front of the trigger guard is wider than the rest and will also need slight grooves for the mag release rivet to slide through.
This is what you should end up with. The trigger guard fits in snug and dose not shift or wobble when it is fully inserted. Next you will need to mark and drill/Dremel the rivet head detents. With the trigger guard pressed firmly into the jig mark the center of each of the five rivet holes. Using the 5/16" drill bit drill a shallow hole for each rivet placement. Using the Dremel with a grinding bit shape each hole to fit the rivet head almost flush.
This is how the trigger guard riveting jig is placed for use.
Next you will make the rivet punch. It is very important the ends be square on this part. Using the piece of .500" steel round stock make a notch .125" deep and 1" high staring .625" up from one end. On the opposite end drill a shallow hole in the center with the 5/16" bit and shape it to fit a rivet head. This will be used to set the trigger guard, rear trunnion and center support rivets.
Now for the last custom jig. This is for setting the rear trunnion and center support rivets. Using the .250" x 1.500" x 8" steel stock place the rear trunnion on it and mark placement of the rivet detents. Drill them with the 5/16" bit and shape with the Dremel. Again you want the rivet head about 1/32" above the jig.
The next step is to rivet the trigger guard/mag catch in place. If you are doing the 80% receiver this is where you will need to drill the trigger guard rivet holes. Using the vise grip pliers position the spacer plate at the mag well opening and lock it in place. Drill the rivet holes with the .156" bit.
Now you will start the riveting process. Attach the trigger guard/mag catch and the mag spacer with the two 8-32 screws and nuts. One screw in the front and the other in the back on the other side. This will keep the assembly tight and straight during the riveting process.
Next insert rivets in the other two holes and put on the trigger guard jig. Place it in the bench vise and snug it against the trigger guard. Use a piece of .250" scrap under the rear rivet hole (Do not use the rivet yet.) and clamp the receiver to the jig. Do not over tighten as this may deform the receiver.
Use the rivet punch as shown to crush the rivets. I use a standard 22 oz framing hammer to do this. You are not trying to destroy the rivet. Go slow and light until you get a feel for the process. Once you have the first two rivets set remove the screws and set the other two. When this is done set the rear rivet and your done.
Now to fit and rivet the front trunnion. You will need to file or trim the front rails to fit the grooves in the trunnion. Go slow. You can take more off but it's hard to add metal back on. Use the trunnion as a guide to mark how far back to file/trim. Once this is done you will adjust the trunnion position.
The sides of the receiver need to set flat against the trunnion. The image on the left still needs fitting. On the right the rails have been trimmed to the proper depth and the trunnion is flush with the front of the receiver.
With the trunnion in place insert and mag. It should fit tight with little or no slop front to back. With the trunnion aligned with the front of the receiver check rivet hole alignment (On 100% or bent flats). In the image on the right you can see that the rivet holes do not line up. They will need to be "Egged" with a file or Dremel. Lock the trunnion in place with a "C" clamp and make the corrections if necessary.
On an 80% receiver blank use the rivet hole locator tool to mark and drill the holes. Lock the trunnion in the receiver with a "C" clamp and mark the holes on one side. Using the .156" bit drill the holes through both sides. This will ensure correct location.
Now to rivet the trunnion in place. Insert a rivet into one of the holes and lock it into a vise with the bucking bar as shown. Use the bolt cutter/rivet tool to crush the rivet. Again go slow until you get a feel for how much pressure it takes.
On the left you can see a flattened rivet. It does not take all that much effort to crush it. On the right you can see the completed rivets. You will notice that I did not take enough time to do the rivet head detent in the bucking bar. Each rivet head has a halo. Since I am making each of these tools as I do this project I have rushed a little. Please learn from my mistakes. This is a defective receiver so it does not matter in this build as I will be taking it down and redoing it again.
The rear trunnion is next. In a 100% or bent flat the receiver will already be at the correct length for placement. I do advise to double check to be sure. If you are using an 80% blank you will need to check the placement of the trunnion. This is done using the dust cover. place the dust cover on the receiver with the front edge as shown. Mark the rear of the receiver where the back of the dust cover is resting.
This mark is where the back of the dust cover channel in the rear trunnion should go. You will need to trim the rails to accept the rear trunnion the same as the front. Once this is done you will need to measure and mark the placement of the rivet holes on one side. Make a test hole with a small bit and insert the trunnion to see if it matches up. Use a round file or the Dremel to adjust the holes. Once the holes are correct use the "C" clamp to hold the trunnion in place and drill through to the other side with the .156" bit. At this point you will set up the receiver and trunnion with the rivets in place in the jig as shown.
You will usually do both rivets from the same side but in this case I am waiting for a delivery of the correct rivets and am using ones that are too short. I did one from each side to give support during this build. Also, please ignore the extra rivet hole at the bottom of the receiver. This was originally going to be a pistol build but the receiver "Went south" on me.
Here is the fully riveted receiver. As you can see this can be done with a minimum of tools and cost. When you compare the overall results with a screw build you can see that this is the way to go.
Next you will need to trim the top rails to accept the bolt and carrier. If you are using a 100% or bent flat receiver this will usually just require a file. On a 80% blank you may need to trim more and make the drop in cutouts. Here is what the finished rails should look like.
You will start with the bolt carrier cutout at the back of the receiver. This cutout is on both sides and goes from the rear trunnion forward .800". Place the carrier on the rails flush against the rear trunnion and mark the front of the opening. Using the cutoff wheels on the Dremel make the cuts just under sized and then file to fit.
Now file the rails until the carrier moves easily back and forth. Next insert the bolt and mark the cutouts on each side. These can be filed in since they are not very deep. Do not over cut these openings.
If you are using a 100% receiver go to the next section. At this point you will need to decide how you are going to attach the rails. The originals are done with a spot welder. This requires a modified lower tong and tip to fit the receiver properly.
Your next step is to install the lower rails and the center support. Using a .250" drill bit as a spacer position the left rail in the receiver. Lock the rail in place using the vise grip pliers. Put a piece of scrap metal or plastic under the jaw on the outside of the receiver to avoid scratching it. Install the bolt and carrier to be sure the bolt will clear the ejector and will lock up in the trunnion and come out easily.
The rails spot welded in place.
The other options are to MIG/TIG weld them or screw them on. To MIG/TIG weld them you will mark the rail location on the outside of the receiver and drill four 3/16" holes for the left rail and five 3/16" holes for the right rail in the receiver only. The left holes should match the centerline of the rail and be evenly spaced. The right side holes are four evenly spaced along the top of the rail and one down by the hammer pin hole. Once the holes are drilled re position the rails and weld in the holes. File or grind to finish and they are done.
To screw the rails on you will use 6-32 screws and nuts. With the rail held in place with vise grip pliers drill the proper size holes through the receiver and rail. Use the same spacing as the MIG/TIG process. Screw the rails in place being sure to use red Locktite. You can also use a little JB Weld or other glue to reinforce the screws.
Next you will install the center support. It will need to be trimmed to fit between the rails. Make the fit is as tight as possible since when you rivet it in place the receiver will pull in a little. This will necessitate filing the rails again for the bolt carrier to move freely. Install the center support rivet the same way you did the rear trunnion rivets. Be sure to go slowly. You do not want to damage the receiver.
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