notebook: autoloading pistol #4

I keep playing around with sketches for an easy-to-build autoloading pistol. Something in a major caliber, with a locked breech.

Most known-workable designs are descendants of Browning's tilting-barrel patent, where the barrel and slide recoil together, and the barrel is pulled down and unlocks from the slide after a short distance. Inertia carries the slide back, and the recoil spring provides oomph to return the slide, strip a cartridge off a loaded magazine, and bring it all back into battery.

Looking at a 1911 or Glock, they're actually fairly simple... but the manufacture is fiendishly complex. It wasn't a problem when designing weapons for military production, but at the DIY level there's just an endless parade of fixturing and cutter grinding.

My #3 design used a MAC-10 style lower, with welded "rail" tabs. Most plastic framed pistols just use some molded-in sheet metal tabs, so welded tabs should work fine. A while later I saw Boris' "Iron Glock"; Boris was approaching the same problem from the other direction, making a sheet metal frame to put a Glock slide on.

Unfortunately, even with a simple sheet metal lower, there's a lot of machine work in the slide. I shamelessly ripped off the Glock/SIG "use the ejection port as a locking lug" setup for #3. There's less machining than a Browning- style slide, but still a lot.

My newest design, imaginatively called "pistol #4", gets rid of the rails and all their machining entirely, and greatly simplifies the slide.

Imagine a MAC-10; a folded U shape lower with a square tube upper, fit inside and held by one or more cross pins. The tube has windows for the FCG, the barrel link, and the ejection port.

The slide turns into a telescopic bolt like a MAC. One or two half-round locking grooves are ahead of the breech, mating to rings on the barrel, like a Tokarev.

The barrel swings down on a link, which is pivoted in a cross pin through the lower.

The pistol would work just like a 1911, except the "slide" reciprocates inside some square tubing instead of on machined rails.

The bolt/slide is the most complicated part, and the only hard part to it would be cutting the locking grooves. Other than that, it's just cutting the barrel channel and the usual extractor, ejector, and firing pin cuts any bolt or slide has to have.

Essentially, it's a square tube gun, except with a tilting barrel to lock the breech.

As usual, the devil is in the details; rear impact surface for the slide, running the trigger bits, being able to assemble all the interlocking bits without having to translate them through four-dimensional space, etc.. I'm still iterating through sketches for that.

Anyway, maybe one of you guys might find a use for the basic idea.

TRX
November 19, 2013

[Does it have to have a locked breech?]

Yes. Blowback designs are well-developed; I'm trying to come up with a "minimum tooling" design for a locked breech. Specifically, no castings, no CNC, and no tool grinder. Hey, it beats watching TV...

TRX
November 21, 2013

[A square tube of thin material forms the bulk of the receiver which the bolt rides in, and an outer "sheath" brazed or welded to the tube reinforces the receiver where needed to beef up the locking recess and load paths.]

I was thinking "inside", but yeah, same basic deal. In fact, putting some of the reinforcement plates outside would simplify things and allow a narrower mechanism.

TRX
November 23, 2013

[What I imagined instead was a U-shaped sheet metal (or even better, channel) piece wrapped over the top of a block of metal, with locking slots cut into the upper web of the channel, and a firing pin hole through the block.]

That's essentially what "Pistol #3" was; the slide would have been a heavy- wall square tube with a separate SIG-style breechblock and a welded-in muzzle piece, and a big square ejection port for the barrel to lock into, also SIG- style, though most people are more familiar with the layout in Glocks and later guns.

The problem with the #3 design was the barrel; the locking lug and underlug, cammed or linked, exceeded available barrel blank diameters. I contemplated a barrel liner pressed into a breechblock, was concerned about the strength of the unsupported liner, then went down the "how to I rifle my own barrel" track, then to welding additional material onto the barrel feet or cam track. All workable, more or less, but I wanted something simpler.

I've been trying to limit the "machine tool" part to the "Harbor Freight mini-mill and mini-lathe" workspace envelope. If you have bigger/better tools available, no problem. Staying within that workspace has killed off a number of otherwise-good ideas...

I don't see a need for any reinforcements in the #4 tubes at all; the barrel tilts down at the front of the upper, but doesn't touch any other part of the frame, other than through the link pin. All of the interactions take place between the barrel and the bolt; think of a 1911 slide, cut away much of it in front of the locking lugs, and put it inside a thinwall tube so it can slide back and forth. No rails; just the abbreviated slide (bolt) and a tube, which is fixed to the lower, where the link pin and FCG are. I need to sharpen my crayons and blow the dust off the scanner.

TRX
November 23, 2013

I thought about that, but if you look at the actual engagement surface of 1911 lugs, it's neither tall nor wide. And the further back to the breechface you move them, the stronger the bolt and the quicker the disconnect as the barrel links down.

If the asymmetry of the bolt ahead of the locking lugs was truly a problem, there's always the fallback of ejecting up, with a bit of an offset on the extractor to keep empties from falling straight down.