this is my longshot. it took me about 2 days to modify and paint it. the mods i did were doubling up the spring with a recon spring, removed both Ar's, and i took out all the safteys. oh and it is about 4 feet long with the extended barrel.

All Steps Viewing
View all steps of an Instructable on the same page when you're a Pro Member.
because Barret makes lots of different firearms, you might be thinking of the M107 or m82A3
I just hung them from the ceiling with wire and spray painted them