I have recently decided get 2 rifles. I would like a rifle with a 26" barrel and another with a shorter(maybe 16" barrel). I was hoping to have 2 separate upper assemblies and use the same lower for both. A .308 would be my ideal platform. I dont think .223 is gonna have the knock i want for distance shots. So far I have decided that I want a POF.308 upper(short) and Rock River Arms .308 Varmint(long). Does anyone know of any problems that may occur with having a setup like this? Or, if piston driven uppers require anything to use with a lower from a diff. company?