Folks are often mistaken thinking they can convert an AR-15 to fire .308 Winchester/7.62 NATO. It can not be done. The AR-15 action and magazine well are not large enough to support the larger caliber.
Below are a CMMG AR-15 STRIPPED LOWER RECEIVER compared to a DPMS LR-308 STRIPPED LOWER RECEIVER
I think the pictures above make the differences obvious. Now in the picture below is a 24″ Bull Barrel AR-15 and a 24″ Bull Barrel DPMS LR-308. Both rifle are built using similar options. Again the differences are obvious. The DPMS weighs in at 16 pounds.