re: ***UPDATE NO.3**My new AR is cycling .223 fine but hanging up on 5.56

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Before you chasing your tail with a headspace gauge that will tell you little to nothing

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It will tell you if it is even safe to fire the thing with 5.56mm ammo. I don't know about you, Clames, but that's a big deal for me.

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does 1/4 of an inch out of battery sound like a headspace issue?

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Not necessarily, but it is something that ought to be checked - ASAP.

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go find another action spring (the misnamed buffer spring) and a H2 buffer to swap in and try 5.56 again. According to PSA you have a standard weight buffer which isn't exactly the best choice for a carbine.

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If a weapon cycles with 1 loading but not another, on a relatively consistent basis, it could be 1 of 1000 things, and the spring is high on the list of suspects - I do not question this. In this case, it's running with a heavier, but faster bullet, and not with the lighter, slower.

But, I would still get the headspace checked before I ran dozens or hundreds of other diagnostic rounds through - but I'm a cautious guy with firearms.