Am looking at purchasing a RRiver lower with Bushmaster A2 16" Carbine flattop...in the photo (normally where I'd post the pic address on my Photopoint site, but it's clogged up), the barrel appears to have 2 section in front of the free-floating hand guard...or a sleeve on the barrel...know what I mean?  It sort of looks like the barel gets suddenly bigger about 6-7 inches behind the end...

what the heck is it?  any ideas???

Thanks!!!

it sounds like it's a M4 A3 version.go to bushmaster.com and you can look at there uppers to detemine what it is.