the 500 mag tube unscrews for sure, pretty easily. mine wasnt loc-tited. try harder? righty tighty lefty loosey
never owned an 835 so cant comment on that.
the stock mossberg 500 follower i had (not sure if they are making different ones now) was a simple blued steel cup, and it had a bit of a rim that could be pinched inbetween the mag tube and receiver
if you werent paying attention and screwed the mag tube back in without pushing the follower in a bit.
regardless its an easy process, and you should do it now and then - most mag tubes ive cleaned were full of rust and gunk.