My rifle has...

[A. F Medic]

The Colt mag says 5.56mm. The barrel says 5.56mm Nato. The mag holder says .223. Whom do I beleive?

There is also MP which I believe stands for Magnetic Particles and there is a C which could be Colt or Chrome

Information

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[LeagleEagle]

All Colt AR types are 5.56mm. The barrel marking controls. C=Colt, MP= magnetic particle tested.

[RCS]

Early SP1 barrel only marked 12

[A. F Medic]

Weren't there ARS made in NATO 5.56mm? Since there are presssre differences between the.223 and 5.56, the higher pressure rifle had to be marked as such?.

Would the only way to prove it would be to do a chamber casting?

[MeatMarket]

It will shoot both. .223 Rem is just the commercial version of 5.56 NATO. Same deal with .308 Win and 7.62 NATO.