Ladies and gentlemen, Farther North Tacticrap is proud to present...
The Pine A.B.O.R.T.I.O.N. is the new choice of the discerning suburban operator for mission-critical tactical plinking operations:
Amazingly, the Pine A.B.O.R.T.I.O.N. actually fires more than once, as shown in this high-round-count remote torture test in our outdoor lab. Granted, the Pine A.B.O.R.T.I.O.N. receiver cracked and the bolt jammed up after firing the second round in the string, and third total round for the receiver, but this sort of reliability far exceeds that required by the typical mall ninja.
Here's a frame-by-frame analysis of our prototype's first and last actual semi-auto operation:
Gun fires. Flash hider does its job too well to see anything.
Bolt flies back, extracting empty case.
The whole remote-test jig also visibly jumps back about 1/2".
Empty case flies out of the ejection port at blur velocity.
Empty case visible as a ^^^ blur in bottom of frame.
Bolt blurring as it closes, stripping the next round from the magazine.
Bolt is back in battery, ready for the second shot.
Boy, wood is sure an ill-suited material for an AR receiver! I trimmed just enough wood to be super happy with the fit of the upper receiver, but my homemade upper mounting pins were a rather tight fit. Rather than file them down in my drill press (the poor man's lathe) like I should have, I instead tried to gently tap the front pin in with a hammer, which instantly resulted in both front upper mounting ears cracking in half.
Ouch! dcorb's prediction was absolutely correct! It's not clear how to fix this, either--you've only got 1/8" of clearance between the pin and the receiver to work with, and this wood just doesn't have the tensile strength to hold the upper down properly. And this is with
a double-width mounting ear compared to milspec, and without
the milspec holes for detent pins, etc. To hold the upper down reliably, I think you'd need a little metal brace of some sort, for example a little sheet-metal piece that wraps around the whole front of the magazine area.
But surprisingly, the rear upper mounting pin went in fine, and amazingly the rear mounting pin alone actually seems to hold the rifle together more or less adequately:
That's when I hit my second major snag. The (surprisingly long) AR bolt is supposed to travel way back into the shoulder stock during recoil, moving inside a "buffer tube" built into the stock. I drilled out a 1" buffer hole in the integrated stock above, but my drill bit wandered pretty darn far off-axis, meaning the upper receiver and buffer tube hole weren't in a straight line.
Net result? The bolt jammed up against the side of the buffer tube about 1" out of battery. I tried opening up the buffer tube a little with a bigger drill bit, but it's almost impossible to straighten a crooked hole without using a boring bar. I have a mill and boring head, but I'd need a way bigger mill to fit the entire stock+receiver under the mill to bore the hole out true.
So I decided to use a more ordinary screw-in shoulder stock with integral buffer tube. Step one was to chop off the integrated stock:
Now that I could fit the smaller receiver under my mini-mill vertically, I used my dial indicator to orient and clamp the reciever on true vertical, and then bored out the buffer tube hole to 1.130":
Now *without* moving the clamping setup or mill, I used the giant 1 3/16" buffer tube tap to tap out the buffer tube hole. I turned it by hand, but used the mill to keep it lined up. If the buffer tube hole goes in crooked, your bolt will jam up while entering the buffer tube hole, so I clamped a little 3/8" pointed rod into the mill for alignment--I'd always wondered what that little hole in the back end of a tap was for! Because I bored and tapped with the same setup, the hole and threads should be (and are!) coaxial.
Buffer tube hole successfully tapped!
However, note how little wood I have left on the top of the receiver--it's less than 1/8", and there isn't room under the charging handle to add much more than that. When I screwed in the shoulder stock, it felt pretty solid, but while posing for the "operator" pix above, I noticed the top of the stock mount was cracked:
Time for some tactical reinforcing steel! (Baling wire!)
The stock cracked in a few more places during firing, until eventually the bolt jammed up after test shot 3. I think the stock mount is under unusually high stress here due to my missing front mounting pin--any tendancy of the upper to pivot upward is stopped only by the stock mount. I'm not comfortable shooting the rifle by hand with this sort of failure, since the stock's buffer tube is the only thing that stops the bolt under recoil.
So, experiment complete.
Good things about a wood AR receiver:
- Wood is light. My stripped receiver + grip weighs 8 oz.
- Wood is cheap. I bought some seasoned hickory for a possible version 2, and it's only about $6 for a receiver-sized piece! (Of course, aluminum forgings are only $22 on sale.)
- It's literally a weekend project, if you've got a mill. Wood mills away very quickly. Even just using pasted-on templates to drill the holes, I didn't have any location problems--FCG, safety, and magazine latch all fit and fired perfectly on the first try.
- Wood makes excellent practice for machining a real aluminum receiver. I learned a huge amount about setup, clamping, and precision while doing this project!
- The magazine well, pistol grip, and hammer and trigger pins all seem to be strong enough to actually use.
Bad things about a wood AR receiver:
- Some milspec features, like the bolt catch, would be very weak, and are best left out. A thin slice of endgrain will usually self-destruct during milling.
- If you let the wood splinter, it looks like crap!
- The upper mounting pin holes and buffer tube hole are both not strong enough, at least when built in crappy pine. There isn't enough clearance around either hole to add much more wood, but some combination of better wood and soaked-in epoxy might bring the tensile strength up to a usable level. Still, I think a metal insert would be needed for long-term reliable operation.
I sure had a blast building and testing out the Pine A.B.O.R.T.I.O.N., I learned a lot both about machining and the AR, and this project provided me with an excuse to get into on the AR platform. Now I just need to find (or build) some decent milling clamps, and I can started milling rifles and pistols from actual aluminum forgings. Finally, I'm also getting excited about building some simple blowback rifle and pistol upper receivers, say in 7.62x25 Tokarev caliber!