No frame stretching, have never had that problem, even with the 300WSM project, but frame flex, which is temporary, doesn't happen with the 500, probably due to the fact that max loads are still only about 51kpsi, not the 60kpsi SAAMI MAP.
BUT, there have been several 500s sent back to the factory with loose barrels due to underlug setback, which happens on other chamberings as well. The underlug is the weak point on a Handi, the 500 may compress the pivot slightly causing the barrel to be loose. I bought a used barrel that was way loose on every one of my late model frames, over 20 of em! I improved the underlug with the same technique as I did with the 300WSM, it's holding up fine with max loads of lil'gun so far. See the FAQ for the underlug improvement.
Tim