I have a DPMS LR-308 and I love it, but it gets pricey to shoot a 308. I hear everyone talking about uppers and lowers. Is the barrel considered part of the upper? The reason I ask is becuase I was woundering if I could buy a 223 upper and use it with my lower and switch my 308 upper with the 223 upper just to save some money on ammo when I am plinking. Thanks in advance!