The case head size of the short mags is .555", the 500 is .526", that's quite a difference in breech thrust even if pressures were the same, which they aren't, the highest pressure you'll find in 500 data is about 52kpsi, short mag data goes above 63kpsi. I tried a 300WSM barrel on a Handi-rifle which is stronger than a Contender, but not as strong as the Encore, I got considerable action flex with just low mid level loads, 150gr @3000fps, yet the same frame works fine with the 500 S&W barrel on it.
Tim