View Full Version : AR 15 with CMMG conversion - hot barrel

September 30, 2012, 08:09 PM
Last week I went to the range with my new CMMG .22 conversion and my Bushmaster carbon 15. I was very pleased with the accuracy at 25 yards all 50 or so rounds in a 2” circle. Today I went back to the range and started out shooting 5.56, maybe 90 rounds so the barrel was pretty hot. I installed the CMMG conversion and started shooting .22 ammunition. The accuracy was terrible; bullets were impacting all over the target. I changed back to 5.56 and my groups were back to normal. Could the hot barrel make that much of a difference with the .22 ammo? Or do I have a problem with my CMMG conversion?

September 30, 2012, 10:26 PM
The lube on a 22lr bullet is wax of some kind, probably you are melting it in the hot barrel. Also, the heat could cause the powder to burn faster,

Just my guess.

October 1, 2012, 06:02 AM
I know the week before when I shot the .22lr first the barrel got barely warm. I guess I could be actually melting the bullet to the point it doesn't track in the rifling. The 5.56 I shot with the hot barrel were perfect.

October 1, 2012, 09:35 AM
It has more to do with the barrel twist rate than anything else.

A "standard" twist rate for a 40 grain .22LR bullet is 1 turn in 16". Most AR barrels nowadays are 1 in 7 or 1 in 9 (there's some 1 in 8 out there and varmint style and older barrels may be 1 in 12 for use with lighter bullets). Those faster twists will overstabilize the bullet and open up your groups a bit. I've read of folks using those conversion kits getting only so-so accuracy at best.

However, there is one other option.

The Aguila SSS 60 grain .22LR stabilizes just fine out of an AR-15 barrel (most .22LR firearms won't stabilize it properly and you get keyholing). It's more expensive than regular .22, but it works just fine. The shorter cases do have a tendency to get stuck in the action but they are easy enough to dig out. Just keep the conversion kit lubed nicely. I use this SSS out of an AR I built for the purpose- with a suppressor it's extremely quiet.

October 2, 2012, 07:58 AM
My Colt has a 1-8" twist. I haven't noticed any unusual heating of the barrel after using the CMMG kit.
Accuracy is iffy. Two inch groups sound just fine.
Gotta try those Aquila SS rounds. I do have a brick some where.

October 2, 2012, 08:22 AM
I had the cmmg AR15 conversion bolt kit but was not happy with the accuracy in a 223 caliber 1-9 twist barrel and sold the kit, bought a dedicated cmmg M4 style 22 rimfire upper with 1-16 twist. The dedicated upper shoots much better, put a Bushnell TRS-25 red dot on and shoot the heck out of it.

October 2, 2012, 07:52 PM
Maybe a couple of things I wasn’t very clear on. When the barrel is cold and I start out shooting .22lr, I have no problem with accuracy. I can put all rounds in a 2 inch circle at 50 yards using the CMMG conversion and the barrel doesn’t get hot at all.

Now if I start shooting .223 or 5.56 the barrel heats up quite a bit but is very accurate but if I switch back to .22lr with the hot barrel the accuracy goes all to heck.

I was talking to a couple of materials engineers here at the research laboratory and they tell me not to worry about the carbon receiver on the Bushmaster because the carbon frame is 40% stronger than aircraft aluminum.

October 3, 2012, 10:07 AM
Maybe a couple of things I wasn’t very clear on. When the barrel is cold and I start out shooting .22lr, I have no problem with accuracy. I can put all rounds in a 2 inch circle at 50 yards using the CMMG conversion and the barrel doesn’t get hot at all.

Just so I understand you properly, this is what I now understand to be what is happening:

1) You shoot some .22LR using the conversion kit. Accuracy is perfectly acceptable.

2) You shoot some .223, get the barrel nice and hot. It shoots fine.

3) You switch back to the conversion kit for more .22LR. Accuracy goes to pot.

What kind of barrel profile do you have? Lightweight? Government? HBAR? I'm still not sure the issue is the conversion kit since it DOES run properly to begin with.

October 3, 2012, 11:08 AM
It's pretty simple. The 22 round heats up very quickly in the hot barrel and the lube fails. Also the powder burns faster and inconsistently. That temp is the only variable here and that temp does, very quickly, effect a little 22 round.

The simplest answer is likely the correct one

October 3, 2012, 06:02 PM
Technosavant, you are correct in the understanding of the problem. I want to say the barrel is a “lightweight” barrel. The rifle is a Bushmaster carbon 15 flat top. The only thing that I can see affecting the accuracy when using .22lr ammo is the barrel temperature. I tend to agree with garyc and his theory. I’ll just have to shoot .22lr with a cold barrel.

October 6, 2012, 10:16 AM
Don't shoot so fast with the .223 and the barrel won't get as hot. Ammo makers love you guys.