I think you might be right on the money on barrel length measurement. In the US they often list AR15 barrels 16" when they are indeed usually 16.5".
Moe
If that is indeed the case, why is there only a 1/2" difference?
The 5.56x45 chamber should add 45mm or ~ 1 3/4" to the total.
1"=25.4mm 45 / 25.4 = 1.7716"
(Not arguing, I just dont know the US regs...)


















































