Had a father and son show up in Vegas, 21-22 May, and the son was shooting an M4 (didn't get the make). I wasn't following his progress but I perked up when I saw his redcoat..... He was getting pretty good scores using the .60 cal rule :o , give or take a bit.
The rifle had several thousand rounds through it (dad thought about 5K but the exterior didn't look like it) and they were shooting 55 grain PMC ammo.
Sure would've liked to inspected the rifle more thoroughly since I've never seen an AR do this.
Any ideas?
RR
Points for the most creative gaming I have ever seen!
Seriously, do you know specifics on the rifle. Twist rate is the first thing that comes to mind after barrel wear. But even with a 1/7 I would not expect to see anything like that out of PMC.
Others have seen something similar over on arfcomm:
http://www.ar15.com/forums/topic.html?b=2&f=145&t=185097
Did you have any heavier or different brand .223 on hand to experiment with?
Techres
Nothing looked out of the ordinary. I've seen crown issues increase group sizes but not keyholing. The M4 struck me as a "consumer grade" that I'd expect to see a 1/9 twist barrel on, not a top tier or custom with the 1/7 barrel. Have never seen an M4 or clone with a 1/12 either. I'll see if I can contact the shooter and if I can inspect/troubleshoot further.
They only had the PMC and while I was packing XM193, Remmy .223 55gr FMJ (shot a 241 with this out of my SPR that weekend), and some 77SMK reloads, I didn't think to run any thru it at the time.
Thanks for the link, it had some interesting ideas.
RR
Had that happen with a Mini-14. Only had about 7500 rds through it, but most of them were VERY rapid fire, barrel was shot out I think.
First symptom was keyholing with 62 gr. ammo, switched to 55 gr and got another 1000 rds or so out of it then started again. Rebarreled it and shoots fine now.
Could not see any visible change to the bore or crown. Toward the last the KH'ing was so bad you would think it was a smoothbore, but still looked fine visually.
SoM
I don't think it was really 55 grain bullets. Something heavier like a 77 or 75 just from looking at the profile on the targets. The best way to tell is to pull a bullet and weigh it.
With 5000 down the tube, the M4 chrome lined barrel is just getting broken in. The only way to truly burn one out is by continuous full auto.
Standard PMC 55 FMJ ammo as far as I could tell. PMC makes a 75 gr target load but I don't think dad would be letting his son "plink" with it.
I'll post up when I get in touch with them.
RR
Well, hard telling just looking at the holes, but my money is on 55 or 62 gr junk ammo (like PMC) and a worn out barrel.
confirm the twist rate (only way to "confirm" is to insert a cleaning rod, screw on a patch tip/patch or brush, pull till it gets tight, mark the rod at top dead center at the muzzle with a marker, pull till the mark comes back to top dead center and measure from the mark to the muzzle = twist rate in inches per one turn_
confirm the bullet weight (pull a bullet, or two, and weigh them - labels lie)
I've seen AR's shoot 55's just fine and throw 62's like bird tracks walking across the target