1/2 scale AR-15

OK, so, after much strife and many distractions over the last few months, I finally found the time and motivation to get back after this.

A few months ago, I conducted the first test fires, with horrific results. The relief cut for the extractor and the feed ramp cut proved far too much for such an energetic little cartridge with thin case walls; both test fired resulted in blown out cases stuck in the chamber, and extractor mangled. At this point, with everything else going on, I just shelved the project for awhile.

A couple weeks back, I finally knocked out another project that had kept me quite busy ( For anyone curious, http://forum.modifiedpowerwheels.com/viewtopic.php?f=19&t=18204), and decided it was time to revisit AR-7.5. First thing was to set back the barrel and re-cut chamber, get rid of those case blowing relief cuts. I also opted to counterbore the barrel for the rim, face off the bolt. Of course, this shortening meant I also had to face the upper receiver, but fortunately, there was enough to still have a half dozen threads engaging. Firing pin also became too short, so had to remake that as well, which I did yesterday.

Initial test fire after those modifications resulted in another blown case head, but less catastrophic, and at least it came out of the chamber. So I knew I was on the right track, but now it was clear that the bolt & buffer assembly were too light. Luckily, I've no shortage of broken carbide cutters around, so I inserted a piece of 5/16 shank into the back of the bolt, and drilled the buffer for a chunk of 1/4" carbide, long as I could get away with. TIG welded them in place, reassembled, test fired again. No case blowout! But the cases were getting caught by the bolt returning home before they could escape the receiver, which is a recoil spring problem. So I clipped 3 coils and stretched it a tad, tried again. The results were positive; out of the next 15 test rounds fired, only one failed to eject. I also tried 5 times with magazine inserted, and 3 of the 5 attempts resulted in a fresh round being chambered (one of the failures coincided with the FTE, the other was a short stroke that pinched the new round).

We are still having issues with hammer power, having only achieved first strike ignition 2 times out of the 20 or so firings thus far. But I am optimistic now!

As an aside, this thing is friggin' loud! Surprised me quite a bit, expecting rimfire pistol report, but my ears telling me something more along the lines of full house .357 magnum.

 

That is too awesome for words. I've been following this thread before becoming a member here. Is there any way to see a video of this thing in action?

 

Glad it's going better for you hope you get it solved soon

 

That's just amazing work, Mach. One of the coolest projects I've run across on the internet.

As an aside, did you ever decide to go through with the 12 gauge mini grenade launcher?


EDIT: I had always just glanced at the pictures whenever this thread popped up, but I took the time to read through all 16 pages. I've got a whole new level of respect for your work now.

 

Thanks for the follow up, can't wait to see you get it running like a top soon!

 

How are you shooting it? Like, physically, how? Vise & a string for both safety and practical reasons, or is it actually large enough to manipulate as a weapon?

...and yes, the magnum rimfires are loud

Congrats on the successful test-fire!

"As an aside, did you ever decide to go through with the 12 gauge mini grenade launcher?"
<He states, knowing full well it's probably already been mentioned> That'd be an issue NFA-wise, but 410 would work, as would a muzzle-loader (or a 410 hidden inside a larger diameter shroud), as would a 22 rimfire blank-powered 'launcher' if I'm not mistaken (don't quote me on that last one)

TCB

 

Not yet, but as soon as I have someone to operate a camera, we'll get a short clip

I just put on eyes & ears and shoot it like the handgun it is. It's diminutive, but still large enough to get a decent grip on. I can't actually get a finger through the trigger guard, though, so really only able to manipulate it by having the sharp lower edge get a bite on my finger tip. Not particularly comfortable....

I was looking at 12 ga flare launchers, the chambers of which are too short to chamber normal 12 ga ammo, so not NFA. The only potential issue is the existence of Aguila mini shells, which can physically be chambered in 12 ga flare pistols (for those who want to make themselves amputees....). I never finished researching the relevant determinations, though.

 

My grandson is looking to become a model maker and intends on making miniature firearms, so I'm curious, did the ATF every give you an answer on the status of your fine AR?

Nice work, and excellent trouble-shooting. You have to be a hellofa machinist.


Cat

 

No, they never responded to my email or my hard copy. I waited almost 6 months, then just went ahead with the actual barrel. Not concerned, would be pretty difficult to make a case that this is a rifle, especially since the codified language is "designed and intended"; I designed and intended this firearm to be a handgun, albeit one that appears to be a rifle if you have no perspective for the size.

 

I actually joined this forum to watch this thread, glad to see it coming together and thanks for taking us for the ride.

 

I'm still in awe.

 

Wow, man. I was in awe of this thread, but that hopped up Power Wheel thread is impressive too. i just "lost" an hour and then found a puddle of drool on my desk. You really are a master craftsman.

...so, is a pintle mounted mini-mini-gun going on the Wrangler?

 

What are the odds of us getting a video of it shooting/cycling at some point soon?

 

Video! Video!

 

This little guy is cheering you on:

 

Amazing little project....

Too cool for school

 

Glad to see you back. I'm also glad that your project is showing you a light at the end of the tunnel.

 

Well, following some of my other builds being completed, I decided to take another whack at this one. I ordered some tungsten to make a new buffer out of (a solid tungsten buffer, not just tungsten weights inside). I also lightened the hammer in order to increase speed and hopefully deal with the light strikes.

Unfortunately, while it drastically reduced the occurrence of case head failures, they still happen occasionally, and we still don't get first strike ignition. I'm really not sure where to go next with this one, as I don't know if I can get enough weight for the energetic little rounds short of making another bolt out of tungsten alloy (spendy-the 1/2" x 2" remnant piece for the buffer was $26!), and even if I deal with that, I don't know what to do about my spring power issue.

I guess I could make a longer buffer tube and longer tungsten buffer to keep the breech closed, but I think I'm gonna have to consult with some veteran gun builders on the spring. I'm not giving up, but there's really nothing more I can do myself at this point for the hammer spring.

Sorry I don't have a more entertaining update for ya'll, but one day we will!

 

One of the issues I and others have run into with retrofitting machinegun designs to operate as semi auto (which involves cramming modified FCG systems into places they were never intended), is that the spring design of the AR simply doesn't store a whole lot of power for its size or volume. A strut-type coil spring like the Garand or 10-22 system is much easier to dial in for a given project (but the strut is in the way of the AR safety)

Titanium firing pin could help while you're ordering expensive metals (did it have a return spring that could be dialed back/eliminated?)

I remember you saying you tried a stiffer coil spring; would it be possible to use a flat coil spring rather than a wire spring? Though totally obnoxious to install due to their strength, these little guys can really pack a wallop with the right dimensions. They are effectively a wire spring with stiffness times however much wider than they are than they are thick;


TCB