I have a bushmaster A3 upper with a 20" heavy barrel on it. I would like to buy a bushmaster 16" M4 profile barrel and M4 handguards and have bushmaster put them on my upper in place of the 20" barrel and full length handguards. Is this something that bushmaster does? What is the cost on gunsmithing services like this and what would be the approx. turnaround on the change.