I've heard a few massive problems with the 556R that make me think it's kind of a terrible idea. Alloy receiver means you can't use standard steel
AK magazines because they tear it up. Occasional weird hammer and headspace issues that make it a jam-o-matic. And the biggest problem is that it will
not work properly with steel-cased ammo, which is what pretty much anybody in their right mind shoots. Overall, I'd avoid it. Especially considering
the reputation SIG USA has.
The AR, in 223 wins nearly all the matches that it is allowed to enter, and many, many top operatives prefer it. If it was ANYTHING like being as
unreliable, fragile, etc, as so many claim it is, the top hands would use something else. But they don't, they use the AR in 223. :-)
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