the fact that having a rifle and machine gun in different calibers complicates the supply problems.
It is just common sense that the more items in your supply system, the more complicated it gets.
We came out of WWII with the .45acp, .30 carbine, and .30-06 are rounds for "long guns" (SMG, carbine, M1Garand, BAR, and Browning .30 MGs).
We went through Korea with the same stuff.
Viet Nam is a transition period, we essentially replaced the .30-06 with the .308, and in rifles replaced the .308 with the .223. BUT, we still had .45acp SMGs, and our ARVN allies were using .30-06s and .30 carbines that they got from us, and I don't think they were making their own ammo for them (if they did, it wasn't in quantity), so they got their ammo from us.
Which meant that ALL these rounds were in our supply system.
I've even heard (from people who were there) that some tanker units went to Desert Storm still using their WWII era M3A1 .45acp SMGs, after we had changed the .45 for the 9mm as our pistol round.
The 5.56 functions as an anti-personnel round ... all attempts to use it as a machine gun round have failed.
If it has failed, what are all those SAW things I keep hearing about??
And there have been
a lot of complaints from current veterans that it is ineffective against insurgents high on drugs.
I have heard this. I have also heard about every other complaint about the .22 caliber being ineffective, since it was adopted. Some of them are quite valid. Others, not so much.
One interesting comment I heard, about today's troops complaining how their rifles were "ineffective" ascribed the bulk of the complaints to the fact that the troops were having to shoot the bad guys 2 or 3 times before they went down.
And how the bulk of todays troops have little experience with real world shooting (before going into combat) and LOTS of experience with video games, where it takes ONE round to drop the bad guys.
We have a lot of troops who have "trained" since their early teen (or pre-teen) years on these "simulators" (video games), and not on real shooting and its results in the field, so they have an ingrained expectation of how things should work (the way they do in the games), and when they find out the real world is different and a bit more complex, they complain about the rifles being ineffective.
Makes sense to me.