Topic: TRX builds the "SG-1"

Retrieved: December 06, 2013

I present the "SG-1", a 5.7x28 bullpup inside an airsoft P-90 shell.

I have a Wells airsoft here and disassembled, a pair of P-90 magazines on the way, a box of 5.7x28 ammunition, and a barrel on the way. I have some O-1 drill rod on hand to make the chambering reamer.

The receiver will be square tubing. Operation will be blowback. It looks like an AR FCG will work, and I'm going to try an AR recoil spring.

Since the airsoft shell is fairly thin plastic, I'm going to extend the receiver all the way to the back and make a buttplate to spread the load. The grip area feels fairly sturdy. I want to have all the parts laid in the shell before I make a final decision on the trigger setup, but it may wind up with having the trigger pivoted off a barrel mount and a pushrod to the back. Maybe I can figure out a way to use a pullrod later.

I haven't found chamber dimensions for the 5.7x25 online, so I'm just going to measure a few of the cartridges, average them, and add .003" per SAAMI allowance.

This one should be a fairly simple build. I expect some fiddling with the bolt weight and spring stiffness to get it to cycle properly. Now we can see how accurate the prediction is...

TRX
March 09, 2011

I measured a cartridge. They don't *look* that straight, but they're .312 at the base and .310 at the shoulder.

I don't have a good way to measure the shoulder angle. I thought about trying to use a flashlight to shine a shadow on some paper and find the angle with a protractor, but after thinking about it, I might just eyeball it. As long as the cartridge comes to a solid stop to set the headspace it might not be a problem. From what I've been reading, some of the P-90s move the shoulder nearly 1/8 inch forward when fired, requiring shoving the shoulder back when resizing, and also limiting case life to three or four reloads. Apparently the "delayed" part of the P-90s action isn't all that delayed...

The bullets do in fact measure .224", as some people have reported.

TRX
March 09, 2011

Quote from: gunguyr on March 09, 2011, 08:03:31 PM
This page has a cartridge drawing... :-* http://en.wikipedia.org/wiki/5.7x28mm

Well, *#@&!! I've been to that page before and completely missed the drawing.

Included angle of 70 degrees 3 minutes... 35 degrees, 1.5 minutes.

Thanks!

TRX
March 11, 2011

The magazines showed up yesterday. I immediately loaded one up. Now I know why the cases have only a .002" taper - with 50, the follower has a slight but definite tilt. With something "normal", the whole magazine would have to bend off to the side.

The function of the ramp mechanism is still mysterious. The tint on both mags is dark enough to make it impossible to see the details. But when you push a cartridge in, the one below rotates the full 90 degrees into alignment with the magazine. I'd expected the ramp would have one or two at various angles. And when you shuck cases out, each one snaps around 90 degrees with an audible click.

Loading the mag is a hassle. There's about a 1/8" area between the feed lips and the bottleneck; the base of the cartridge you're putting in has to go in that one place, and you have to push straight down, then back. It takes more pressure than I expected. I don't think I could do it if my hands were sweaty or very cold.

TRX
March 12, 2011

I found this chart in another thread here:

These guns are all 9x19, .45, or 7.62x25. The available bolt travel in the P90 buttstock is a bit over 5 inches. The muzzle energy of the 5.7x28 is 394 ft-lb. It matches best with the Uzi at 4.33 inches of travel and 382 ft-lb, next best to the PPSh41 with 3.35 inches of travel and 560 ft-lb. Apex has Uzi springs for $10.00. They don't have any AR trigger groups, though, so I'm going to hold off while I look for a place that can sell both so I can save on shipping. Maybe, anyway.

My SWAG that the 5.7 would be similar to the 9x19 or 7.62x25 seems to have been close.

TRX
March 18, 2011

I'm watching an FCG on GB. Maybe it won't go up too much. If it does, I'll punt and try one of my spare AK FCGs.

The barrel just showed up. This weekend's task: chambering reamer! People keep talking about how easy D-bit reamers are, so I guess I'll find out...

Zeb
March 19, 2011

Here you go ( http://www.prexis.com/sten/homebuilder/index.php?topic=15826.0 )

TRX
March 19, 2011

Thanks, Zeb! I hadn't noticed that thread.

Interestingly, the higher AR57 bolt weight is near the bottom of the SMG bolt weight chart I put up earlier. Only the PPS43 is lighter. 20.8oz for the AR57, 24oz for the Uzi, I should be right in there. I figure it'll be easier to take weight off than to add it.

TRX
March 20, 2011

I haven't weighed the barrel blank yet, but a 25" steel bar 1-1/4" in diameter is freakin' heavy. The .223" hole down the middle is irrelevant.

At $50 I can't gripe any, but Mr. Lathe is going to make a lot of passes down that barrel before it's the right diameter.

TRX
April 05, 2011

...and two weeks later, Mr. Lathe is still at it... well, I haven't been able to work on it every day, and the cutter wants to chatter even though I've played with the geometry a few times, so I'm only cutting .004" or so per pass. I couldn't justify $150 for a steady rest from eBay or the time to build one, though I'm certainly going to need one in the future.

On the other hand, there's a truly impressive pile of chips in the tray. I figure just over half of the blank will wind up there in the end.

I also have an 8" cut-off that I can use for playing with the chambering reamer, which will relieve a lot of anxiety...

TRX
April 12, 2011

Ooo-kay! The main part of the barrel is now reduced from 1.25 to .625". Now I need to whack the chamber area down to 1.00". I'm using the basic M-16 dimensions, which ought to be more than sufficient for the 5.7 cartride.

Rather than spending the time to cut a smooth blending curve, I'm just going to cut a couple of rounded steps like an old Mauser bolt action. Not as pretty as a curve, but nobody will see it anyway.

TRX
June 12, 2011 The barrel blank.

The airsoft shell, opened up. It was glued at the muzzle, and the mag release apparently snaps in place and was a bugger to get apart.

P90-shell-open-2.jpg
TRX
June 13, 2011

I'm going to put a strut between the receiver and a load plate ahead of the plastic buttplate. There shouldn't be any direct loads on the plastic shell, other than just holding it. It seems sturdy enough for that.

I haven't decided how I'm going to hold the receiver to the shell. The thought so far is to use the buttplate and a bushing at the muzzle and let it float.

[magazine workings]

TRX
June 18, 2011

The plastic over the naughty bits is smoked enough to make it hard to see, and the patent drawings are only "representative" of the actual magazine... but it does work.

The weirdest thing, and one that's not immediately obvious, is that each cartridge turns the full 90 degrees each time it is presented to the feed lips. I had thought there'd be, say, one intermediate cartridge at 45 degrees, or something. Instead, each one goes "snap!" into position... and the follower is flat, no dummy cartridges forcing the last real one into position.

I bought a real FN mag and a Korean clone; other than a couple of molded numbers, they're impossible to tell apart.

TRX
November 17, 2011

I might as well formally bow out of the contest. The SG-1 was intended to be a simple blowback build, no problem there, but way too much "Real Life"(tm) came up to submarine things.

Not to worry, I'll finish it up eventually. It's for my wife, and she periodically asks how it's coming along. I tell her it would be done if it wasn't for Extreme Remodeling on the house...

I also keep having to move the box of parts for the Imperial Battle Rifle around to get room to work on other things. That sucker will get finished eventually, too.