The "old Stoner" 63As were on USN arsenals until 1982, and there are plenty of old weapons still lying around.
Caliber change has been sidelined at the moment, but telescoped and plastic cased ammo are in vogue. This would only affect the amount of ammo a squad would be carrying in game though.
There have been a lot of weapons projects happening in the last few years too in terms of small arms.
You might want to look into the following to check for accuracy:
XM320/M320
XM26
XM25
XM312/XM307K50
XM307
XM110/M110
Mk 12 Mod 0/1 (SPR)
Mk 13 Mod 0/1/2 (the rifle)
EX 13 Mod 0 (FN EGLM candidate, submitted with the SCAR-L/H)
Mk 14 Mod 0
Mk 15 Mod 0/M88 PIP
EX 16/Mk 16 Mod 0
EX 17/Mk 17 Mod 0
Mk 18 Mod 0 (CQB-R)
Mk 47 Mod 0
Squad Advanced Marksman Rifle (SAM-R)
Designated Marksman Rifle (DMR)
Squad Designated Marksman Rifle (SDM-R)
I would recommend browsing this site for more info on stuff in development:
http://www.dtic.mil/ndia/