New to the platform it appears to be easier for a build with DPMS. I've built 2 ar-15s chambered in 6.8 spc II both run flawless. Looking to step up for the big brother in 308win & 6.5cm. Need one lower and two uppers 16"308 lightweight for hunting and 24" 6.5 for long range. Can I use one buffer & spring in a lower when changing the uppers out?