Gunnit Rust: .22lr AR-15 build from trigger jig (rather than an 80% lower), AKA "the Lyndon build" (Tier II)
Before we get started, here are links to the good stuff:
A while back, user backbencher on ar15.com got me a trigger jig as a gift in celebration of his wedding. This post is a crosspost of a long overdue update that I also made on Arfcom on the Build It Yourself forum.
Over the last year, I've made a couple variations of the Lyndon Build. The Lyndon build, ironically named after President Lyndon B. Johnson who was fond of serial numbers, is a gun build with the core idea being that the build, when disassembled, has no parts that are considered a firearm. It is distinguished from an 80% build because with an 80%, once you complete the receiver, the receiver is still a firearm. Theoretically (though not tested in court or determined to be such by the ATF), a disassembled Lyndon build has no firearm parts, rather, instead of having a receiver once disassembled, you are left with a trigger jig and a pile of other parts.
My build is not quite as technically a Lyndon build as backbencher's, especially his AR 5.7 pistol build which did not require modification to the trigger jig at all in order to create a functioning semi-auto, centerfire firearm, because my build required pretty substantial modification to the trigger jig, thereby rendering it no longer a trigger jig (I'm quite certain that if I dry fired on the jig at this point, it would crack). Thus, while still referring to it as a Lyndon build, it's more like an 80% build--perhaps we will call it a 60% build or a 0% build, whatever you like. But the point of it is that I was able to create a functioning semi-auto .22lr AR out of a trigger jig and a magwell from a polymer lower with nothing more than a hand drill, a hacksaw, and a Dremel.
Here are some photos of the process, summarizing the build...
I started, again, with just a trigger jig as linked above.
The first thing I did was drill a hole for the takedown pin spring and detent. I went from the top because reasons. I went all the way through the bottom so it literally doesn't matter.
Next, I had to lop off some extra material from the front of the trigger jig so that we could start fitting a magwell to it.
Removing this material is what renders this no longer a trigger jig, since the front where the hammer hits is now super thin and unable to hold the rubber hammer stop that comes with the assembly.
Then, I took out my trusty Dremel and sanded down the front where I hacksawed off a piece, just to make it nice and smooth. I didn't get a picture from this process while I was doing it, but here's a recent photo to show you.
This front side needs to be smooth so that the mag catch doesn't snag on any roughness.
Now it was time to fit a magwell.
Long ago, I had called u/tnarmsco to buy some magwells from demilled lowers so I could practice RIT dying them. I still had them lying around so I decided to use one of them for this project. I simply took a hacksaw to the magwell and cut it down so that it would fit against the trigger jig when both the jig and the magwell were pinned into their respective holes on an upper receiver. I did fine tuning with a dremel, and as you can see in one of the photos above, I had to dremel down the mag catch and button in order to have the magwell fit against the jig. This also meant I had to destroy the provisions for a bolt catch, which doesn't really matter since I'm building a .22lr anyway. You can see how much of the mag catch I had to sand away by looking at the size of its empty hole here. You can also see why I couldn't install a bolt catch. You can also see why I couldn't file down the trigger jig any further, because if I did I would have taken away the entire front of the trigger jig and the tab that holds the hammer pin would have fallen off, rendering it nonfunctional.
I had originally intended to somehow permanently join the magwell and the trigger jig along the yellow and black dashed line in this photo.
I eventually decided against it because that, without a doubt, would form a lower receiver. I figured the two pins in the upper receiver held the two pieces together well enough. What I was left with was something of a .22lr pistol that looked like this.
After I installed the sanded down mag catch and button, this pistol functioned fine in this state, but I still wasn't totally happy with it. Note that it uses a S&W M&P15-22 magazine. I use the Redi-Mag/Booniepacker magazine adapter in order to use these mags because I like loading them better, but the normal CMMG mags work too without the adapter.
To improve the pistol, I decided to add a brace.
I didn't get pictures of me installing a brace, but you can see the "finished" product above. Basically, I took a leftover keymod picatinny rail, sanded down the keymod bump on the back of the rail (you can see it in this photo), cut the rail to length, and drilled two holes in it for screws to mount to the back of the trigger jig. I then drilled two holes in the back of the trigger jig to accept the screws. I just used whatever random ass screws I found in my garage. They aren't even the same. I didn't tap the holes for the screws, I just drilled the receiving holes just slightly under the diameter of the screw threads and self threaded them in by mashing on the back of my drill as I screwed them in.
I used two screws so the rail wouldn't rotate. It isn't centered very well. Oh well, good enough.
After I added the rail to the back, I needed a way to attach a buffer tube and brace. Well I knew you could get a picatinny/buffer tube adapter, so I went out and bought one. I attached the adapter to the rail and screwed in a pistol buffer tube, then added a shockwave brace. Voila.
I sold the 16" CMMG upper and build a 4.5" CMMG upper. With everything put together, I was left with this:
You can see that the trigger jig got a little warped and beat up in my vice. Who cares, as long as it runs!
And run it does: Youtube video
Now that the .22lr AR pistol is done and functioning reliably (we shot about 400 rounds through it without any malfunctions except for rimlock once in the magazine), I'm going to be working on the other trigger jig that accidentally got sent to me when backbencher bought me the first one.
So far, this trigger jig has only been slightly cut on the back end so that it could be fit up against the Hera Arms stock, but it has otherwise been unmodified.
Here is a top-down view of the two next to each other so you can see how much material I actually removed from the front of the trigger jig for the .22lr build. Sorry they're not perfectly lined up, but you can get an idea.
The next trigger jig will probably also have to be heavily modified because I plan on making this one into a 5.56 rifle. I will post that one for the next gunnit rust!
Posted Sep 23, 2018, 9:59 PM UTC