View Full Version : AR caliber

October 22, 2012, 03:42 PM
Picked up my first AR a couple weeks ago. Private sale, got 6 mags and about 600 rounds of .223 ammo.

It shoots .223 fine. But am curious about 5.56x45

No barrel markings anywhere except "C MP C" stamps the underside between the front sight and the flash suppressor.

I'm 95% sure that the upper is completely Colt (model 602 I think) - but without a marking on the barrel to tell, how to know which caliber?

ETA: no markings under handguard either:confused:

Crow Hunter
October 22, 2012, 03:56 PM
If it is a Colt, it is most likely a 5.56 chamber.

Even their "target" rifles have 5.56 chambers. (Modern ones anyway)

I wouldn't worry about it.

Worst thing that would happen if you shoot high pressure 5.56 in a tight .223 chamber is you "pop" a primer back into the trigger group and it locks up your trigger.

It won't blow up or anything, there isn't that much pressure difference between them.

October 22, 2012, 04:07 PM
found this on another site:

Barrel Marked C MP C. C meaning Colt, M meaning Magnetic particle inspected, P standing for Proofed, and the C marking indicates a chrome chamber

from the other threads out there, your Colt is chambered in 5.56x45mm

October 23, 2012, 08:05 AM
Cool, that's what I was hoping for.

<-------AR Newb, does it show?


ETA: Long as I'm at it, anyone happen to know the twist rate offhand, or how I might determine it myself?

October 23, 2012, 08:33 AM
doing a google search, everyone out there is saying yours is a 1:7 twist

October 23, 2012, 08:41 AM
Ok, so 1 in 7" on a 20" barrel. Considered good? Seems pretty accurate with 55 grain .223. Would that be the optimum bullet weight?

October 23, 2012, 10:13 AM
Buy a box of several bullet weights from several manufacturers. The rifle will tell you what it likes.

October 23, 2012, 10:40 AM
Buy a box of several bullet weights from several manufacturers. The rifle will tell you what it likes.

Very true. Likely it'll be 62gr and up but one of my PSA guns (1 in 7) gets just as good groups with 55gr as 75gr. Every barrel really is unique.

October 24, 2012, 11:06 AM
It did pretty well with the 55grain - I was doing a function test more than one for accuracy. Grouped quite well at 50 yards considering I wasn't really trying that hard.

May have to try some 62s and see what happens.

Thanks for the barrel info