There are two big companies out there that do a lot of the milsim stuff - SpecOps paintball, and OpsGear. You can easily fins what you are looking for.
A quick look found this: http://shop.specialopspaintball.com/...-magazine.html
for your M4 style gsa through mag, which will run around $40. If you look through you can find shrouds as well, with or without rails.
2003 ICD BKO w/ 45/4.5k, Ricochet Apache
'98 r/f Autococker w/ 45/4.5k, J&J Edge kit (14"), Ricochet 2KX
PCS US-5 Mech. w/ 48/3k, J&J Edge kit (10"), cyclone feed, JCS folding voodoo stock with RVA
Kingman Hammer w/ 13/3k, J&J Edge kit (14")
Projects in the Works:
'99 r/f Autococker w/ 45/4.5k, J&J Edge kit (14"), and THE CYCLONE!!!