The set up that I have is:
BT-4 Combat
BT-1913 20" Barrel
BT F.E. Collapsible Stock
BT Adjustable Sight Rail
BT-4 Magazine Expansion Chamber
Ricochet 2K Hopper
Remote
4500 PSI Tank
View of BT-4 Combat with my BT-16 in the top of the picture
Closer picture of the 20" barrel
Close up of back half
Close up of the magazine expansion chamber
Any comments or thoughts on this set up?