partsjay........I wouldn't shoot a reagular mechanical with your set-up. You could should a Rocky Mountain Snyper Mechanical or a Rocket Steelheadbut otherwise I'd stick to fixed. You really want to have a High KE (Kinetic Energy) to shoot a expandable head. I think Spitfires are 1 3/8" diameter. You definetely wouldn't want a bigger diameter head. I like to see people have a KE over 60 to shoot mechanicals. I'm guessing your rig isaround low 50's for KE. If you can weigh you arrow(in grains)and get you actual F.P.S (feet per second) you can find out your KE.

GoodLuck!