AR caliber

Stressfire · 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

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.

pturner67 · 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

Stressfire · October 23, 2012, 08:05 AM

Cool, that's what I was hoping for.

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

Thanks!

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

pturner67 · October 23, 2012, 08:33 AM

doing a google search, everyone out there is saying yours is a 1:7 twist

Stressfire · 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?

Achilles11B · October 23, 2012, 10:13 AM

Buy a box of several bullet weights from several manufacturers. The rifle will tell you what it likes.

Mrgunsngear · October 23, 2012, 10:40 AM

Quote:

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.

Stressfire · 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

October 22, 2012, 03:42 PM	#1
Stressfire Senior Member Join Date: June 16, 2011 Location: Ohio Posts: 1,497	AR caliber 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 __________________ "The best diplomat I know is a fully charged phaser bank" - Montgomery Scott

October 22, 2012, 03:56 PM	#2
Crow Hunter Senior Member Join Date: January 27, 2012 Posts: 1,078	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. __________________ I am no longer participating in gun forums. Good luck.

October 23, 2012, 08:05 AM	#4
Stressfire Senior Member Join Date: June 16, 2011 Location: Ohio Posts: 1,497	Cool, that's what I was hoping for. <-------AR Newb, does it show? Thanks! ETA: Long as I'm at it, anyone happen to know the twist rate offhand, or how I might determine it myself? __________________ "The best diplomat I know is a fully charged phaser bank" - Montgomery Scott

October 23, 2012, 08:41 AM	#6
Stressfire Senior Member Join Date: June 16, 2011 Location: Ohio Posts: 1,497	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? __________________ "The best diplomat I know is a fully charged phaser bank" - Montgomery Scott

October 23, 2012, 10:13 AM	#7
Achilles11B Senior Member Join Date: November 18, 2010 Posts: 595	Buy a box of several bullet weights from several manufacturers. The rifle will tell you what it likes. __________________ Good equipment will never be a substitute for good training.

October 22, 2012, 04:07 PM	#3
pturner67 Senior Member Join Date: February 26, 2006 Posts: 737	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, 08:33 AM	#5
pturner67 Senior Member Join Date: February 26, 2006 Posts: 737	doing a google search, everyone out there is saying yours is a 1:7 twist

October 24, 2012, 11:06 AM	#9
Stressfire Senior Member Join Date: June 16, 2011 Location: Ohio Posts: 1,497	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 __________________ "The best diplomat I know is a fully charged phaser bank" - Montgomery Scott