ive searched but had no luck getting a good answer. anyway, i was looking at cmp's website and they have the U.S.M.C 40x receivers for sale for 275. theyre barreled but with no trigger or bolt. so my question is...where do i find a bolt and also, what has to be done to it to make it work in the receiver i get? is there a headspace that has to be set with it? im new to the 40x world and just want to learn what i might be getting into before i lay down the money.