I just remembered something... one of the problems with using
the M16 in vietnam was that the M193 round fragmented even when
going through light cover, i.e. even leaves and small branches will
protect the target. I am not sure about the M193 rounds now, but
using the M4 will make this problem more prevalent versus the M16
or SAR-21 with the 20 inch barrel. Anyway, its not that big a jump
from the M4 back to the M16, so if they encounter such problems I
am sure they can just manufacture M16 upper receivers to retrofit
their M4s.
Actually fragmentation will be less likely from the M4 due to
the reduced muzzle velocity. This is because the bullets fragment
due to the energy overcoming the mechnical strength of the round
after it enters a dense medium, but if you do not drive the round
fast enough, this effect will be reduced.
Hence from the M4, the "sweet" spot of the 5.56mm where it
causes the most damage is significantly reduced compared to a full
length barrel. Hence the noticable weakness of the M4 compared
to other firearms shooting the same ammo.
