But, seriously, why? There is a reason weapons have evolved FROM battle rifles TOO assault rifles. Assualt rifles weigh less, the ammo weighes less and is smaller so you can carry more, and the average range of a military encounter is within 300m (less so for urban, i.e. shadowrun-like).
This seems more about damage and meta gaming, IMHO.
I'd point out that in the past few years, there has been a movement BACK to returning Battle Rifles to service, having at least one or two in each squad for longer range engagements or firefights where the ability to punch through light cover is important. The US has actually been taking old M-14s out of mothballs and re-issuing them.
The Assault Rifle was developed specifically for warfare involving troops operating in long patrols far afield from supply, making ammo capacity and weight a big issue. Additionally, as you mentioned the fights were taking place at closer ranges. In Vietnam and Korea especially the environment reduced visibility severely, with many fights being initiated at nearly point blank range as one or both sides virtually stumbled into each other.
The past few wars, troops have been in fairly close placement to supply areas, and on short patrols, making the weight and capacity issue much less of a problem. Fights have been occurring at longer ranges and with much more hard cover due to the urban environments. So a harder hitting larger caliber round is in order.
-k
