>>7679366>this triggers the smooth brainsOpen & Play Big Cannon is pretty decent and they've dropped to about $36 average on eBay, as long as you avoid TFSource overcharging for it. Just get a 6mm drill bit to make a bicep hole and you're good to go, unless your skull is concave and you side mount.
MFT Megatron is also pretty good for the price, what with official deluxes being $20+ now. No mod needed, the arm peg works fine. I'll use the side peg to build a G1 toy-style black arm bracket.
