OP
OP
MachIVshooter
Member
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.
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.
