For my next project I'm planning on getting a clone reciever and parts and having them built by someplace like IGF. But I can't decide what caliber, or whether to go long or short barrel. I already use .223 and 9mm so hk93/53 or hk94/mp5 style guns would keep me from having to start collecting a new caliber. On the other hand, maybe it would be worth it to have a full-size rifle caliber, and the .308 mags are a lot cheaper. If I go for a 9mm I'm definitely going to file the SBR paperwork - not sure with the others. If you were going to build an HK pattern semi-auto rifle which would you build? Which caliber do you think works the best in an HK style rifle?