My opinion is that this is just bunk being spread by a non-handi owner / shooter. At 500 - 1,000 rounds it should just be shooting to it's best, the barrel just being well polished. 500 rounds of excessively heavy handloads can stretch a frame, but properly assembled and tuned loads, or factory loads, will not stretch the frames. (Having a 308 on an SB-1 frame will likely also cause frame stretch)