Nothing showed up in a forum Search. so thinking about getting a pistol carbine kit for CQB (for TM M9 or Hi Capa based) and want to run an extended barrel, is there a rule of thumb or calculation for increase/decrease of FPS relative to barrel length or is it just a case of fit it and see? If I put a 300mm + barrel is going to be a likelyhood of it running hot or not?