I just picked up the July issue of "GUNS & WEAPONS for law enforcement". They have a pretty extensive article about Barrett's new M468, 6.8x43mm SPC rifle. Its pretty much the same "rifle" as the M16A2/M4A1 with the "major" difference being the new Remington 6.8x43mm SPC cartridge. The best part is to make the rifle you only need the "upper half" and a new "magazine" if you already have an M16A2/M4A1 or an AR15A2. It looks like a promising replacement for the 5.56x45mm or 223 Remington.
Here are the ballistic's from the article-
5.45mm
M16A2 with a 20 inch barrel and 62 gr. bullets- 2900 fps, no fpe given.
M4A1 with a 14 inch barrel and 62 gr. bullets- 2600 fps, no fpe given.
7.62x39mm
Ak47 with a 16 inch barrel and no bullet weight or fps given, 1445 fpe.
6.8x43mm
M468 with a 16 inch barrel and 115 gr. bullet- 2600 fps, 1728 fpe.
M468 with a 12 inch barrel and 115 gr. bullets-2400 fps, 1472 fpe.
looks pretty impressive! :-D