9-Hole Reviews takes on the flagship AR-15. The AR by which all others are measured as acceptable or as wanting. The Colt (or FN) M4A1 is the current standard issue rifle for US ground forces. The Marines and Army both consider the standard. The Marines also use the M27 and consider the M27 superior in many respects, but that simply makes it an exceeds expectation platform. The same can be said of the M4A1 Block II and Block III/URG-I rifles. They exceed the standard set by the M4A1 Block I/ M4A1 service rifle.
The original M4, even the railed versions, and M16A4 actually had a fairly dramatic performance gap. The Marines did not switch to the M4, despite the mobility advantages, until much later in the PIP phase than the Army did. The reliability gap especially for stoppage rates was not what the USMC wanted to see. Through some upgrades to the gas system, magazines, ammunition powder burn rates, and recoil system, the M4A1 closed the reliability gap with the M16 until the primary difference between the too was muzzle velocity. The M16A4 still throws a faster and therefore further 5.56 round, but the odds of that round going off and properly cycling are finally on par.
Josh and Henry do the voodoo that they do and see what the rifle, as issued with an ACOG, can do in practical accuracy.