Anyways, the M3 - which is the *standard* DoD mag for M4/M16 rifles across all services and agencies
I think they're technically *a* standard (along with the STANAG 4179) specifically because the M2s don't work in M27s. The M2s were being used unofficially, but the spring pressure isn't strong enough for use in any of the -416 derivatives because of case jump in the mag. The EMAG addressed this, but it wasn't compliant with 4179, and thus begat the M3 to work in both the Stoner and HK designs.
M3s have been utterly reliable in my 416s, but I will say they are thicker, so if you have a tight mag well and/or start to get dirt/sand on the mag/into the mag well, the standard seat/pull/push check is even more necessary.