To my mind the M1 and the M4 are very similar in a sense-
Both were developed for units who were issued something other than battle rifles, and both have found favor along a far broader base than originaly intended. Both have became favored weapons of SF using a wide variety of "add-ons" and both have had some controvercy about "knock down power".
I was in the first cycle of 19Ks (tankers) at Ft.Knox to qualify with the M4 instead of the M16A2 and we found that there were occasions when the M4s wouldnt knock over a pop-up target on the first shot either. Now I surmise that this is because the popups are thin plastic shells, and only absorbed a small percentage of the kinetic energy of the round, but I also know that the pop-ups we shot with the M240, I could squeeze off one at a time and I never, ever had one fail to go down on me. Dont get me wrong, I'm pleased as punch that I've got something other than the M9, and VERY happy I dont have to carry a Grease Gun or an M1 Carbine into combat, but I just wish the Army would develop and employ ammo for the M4 specificaly. Maybe soft points or something.
What I do know is, if I ever have to fight outside my tank, my first grab is going to be for a 7.62x39mm or 5.45x40 weapon- I qual once a year with an M4, and shoot it maybe three times a year. I put literaly crates of 7.62x39 and 5.45 down range for every shot I fire with 5.56. Besides, I've been shooting the AK and the SKS since I was 14, I'd like to imagine my ability to operate it while under extreme stress will fare better than my ability to operate an M4 that I'd forget how to shoot while qualifying....
Ulimatly I think the best weapon for a dismounted tanker would be a Mini-Thirty or an AKMS. Just my personal feeling.
RG