Machining an AR15 Lower Forging - August 26th, 2016

Back in late February I stumbled across a guy selling raw M16 lower forgings on Gunbroker. AR15's have never been my cup of tea, but the machining challenge was too good to pass up. I've looked at 80% lowers in the past, but there wasn't enough challenge in it to offset my general lack of interest in the gun itself. But turning a raw forging into a working gun sounded interesting, so I shelled out $14 (free shipping!) and started working on it. I found an amazing set of instructions written by Ray Brandes here that I followed. 80 pages detailing exactly how to machine the forging.

Raw AR15 Lower Forging


There's not much sense in having a functional lower without an upper so you can at least shoot it, so I bought an M16A1 parts kit from M&AParts and a 20" light weight barrel from Del-Ton with 1x9 twist. I prefer retro AR's with carry handles to modern ones covered in rails.

Raw AR15 Lower Forging


The mill vice isn't used at all in this project as it's a lot easier to clamp the forging directly to the table or to an angle plate. So the first thing to do is make a set of clamping pads that will allow the forging to be clamped with even pressure. Here I'm using a flycutter to surface some aluminum blocks to .500" thick. Incidentally, that flycutter was the first project I made when I bought my mill.

Raw AR15 Lower Forging


Here are the instructions, completed clamping pads, and the raw forging.

Raw AR15 Lower Forging


First I used the right hand side clamping pad and clamped the forging to the table. When dealing with forgings or castings, nothing is perfectly flat so I just indicated the forging until it averaged parallel to the table. The first cut is critical in establishing a datum that will be used throughout the project to locate other features.

Raw AR15 Lower Forging


Next I came in with a 1" diameter endmill and milled the surface that mates with the upper flat. I also did the radius, top, and back of the buffer tube tower.

Raw AR15 Lower Forging


Here I've clamped the forging to an angle plate and milled/drilled/reamed the required features on the left hand side. Pivot pin holes, mag catch slot and hole, trigger/hammer/selector holes, selector bumps, bolt release hole and slot, trigger guard hole, grip tang...

Raw AR15 Lower Forging


Then I flipped it around and finished the features on the right hand side. Milling out the clearance hole for the magazine catch spring was tricky. You've got to mill down to just .054" before breaking through the other side. Not much room for error.

Raw AR15 Lower Forging


Next I stood the forging up to work on the rear. Here I bored the hole for the buffer tube to 1.125". I will thread it on my lathe later, because there's no way I'm spending $50 for a 1 3/16-16 tap that I'll likely only use once.

Raw AR15 Lower Forging


After the magazine catch spring, this was the next nerve wracking operation. The drill bit is a long 3/32" bit and there's no way you could accurately drill this hole without the drilling guide block outlined in Ray's instructions. This operation ended up not being as big a problem as I thought it'd be.

Raw AR15 Lower Forging


Then I flipped the forging around and milled out the front takedown pin lugs. I also drilled the hole for the detent spring that holds the takedown pin.

Raw AR15 Lower Forging


So far everything has been easy, but the next few setups required a protractor. I tipped the forging over at 60º and flattened out the grip tang. Then I drilled and tapped it 1/4-28.

Raw AR15 Lower Forging


Then over to 6º to drill the buffer retainer detent hole. I wanted to wait until after this step before threading the buffer tube tower as the threads could make starting the hole harder.

Raw AR15 Lower Forging


Next I tipped the lower up 15º to cut a tiny chamfer on the top of the buffer tube tower to prevent the charging handle from getting stuck.

Raw AR15 Lower Forging


I'm going to thread the buffer tube tower in my lathe, but I need a way to chuck the forging. So I drilled and tapped two 5/16-18 holes in what will become the magazine well and fire control group pocket.

Raw AR15 Lower Forging


Now I could bolt the forging onto a square bar and center it with a 4 jaw chuck. It took a fair amount of finagling around to get the forging square to the lathe's ways and running concentric. Threading it was just like any other ID threading job.

Raw AR15 Lower Forging


Now, on to the dreaded magazine well. First I drilled the corners of the magazine well with an 1/8" drill bit, then drilled out the waste with a 1/2" bit.

Raw AR15 Lower Forging


Next I came in with a 3/4" endmill and hogged out most of the waste. In retrospect I probably should have skipped the 1/2" holes because there was a phenomenal amount of chatter. Or perhaps a 3 flute end mill would have worked better than the two fluter I used.

Raw AR15 Lower Forging


Then I used a long 3/8" endmill to get what I could. There's still a few ridges left in the corners where the radius of the 3/8" endmill couldn't reach, so those will be taken out with hand files. Although, this could be a good excuse to start cruising eBay for a hobby shop sized shaper...

Raw AR15 Lower Forging


After 15 or 20 minutes I had the ridges knocked off and magazines fit freely! There's not much rattle, but they fall out easily. I also stamped an "S" and an "F" for "Safe" and "Fire" in the appropriate place.

Raw AR15 Lower Forging


After milling the slot for the bolt catch I basically had an 80% lower. Then I milled out the fire control group pocket and slot for the rear takedown pin. At this point my forging turned into a full blown firearm in the eyes of the ATF.

Raw AR15 Lower Forging


Finally in the last setup I machined the features on the bottom of the forging. I cut the slot for the trigger guard and drilled the hole for the selector detent.

Raw AR15 Lower Forging


Here's the finished lower, clamping pads, and my trusty instruction booklet looking a little worse for wear. I spent half an hour or so filing out the rough forging seams that were left.

Raw AR15 Lower Forging


Finally I got all the parts put in the lower and my surplus M16A1 upper mounted.

Raw AR15 Lower Forging


Then it was down to the gun range to try it out!

Raw AR15 Lower Forging


Raw AR15 Lower Forging


My first group of the day at 100 yards are the wider three shots and they measure 2-1/8". Not bad. I then shot 3 or 4 more three shot groups and came back to my first target to see what the group was like now that the barrel was warm and I was more in the zone. A 3/8" group!!! At 100 yards! With the cheapest Russian ammo I could find! Economy grade barrel and trigger! Iron sights! I don't shoot bench rest very often, so I am beyond thrilled with that group. Unfortunately, now that I know the gun is accurate, I won't have anything to blame my misses on...

Raw AR15 Lower Forging
.36 MOA, 100 yards, 55 grain Tulammo, stock M16A1 iron sights


Deer #24 AR15
Accurate enough for deer hunting, anyway!


Deer #24 AR15
And cold weather hunting!