Can you shoot slugs in a 835 mossberg?

No. The 835 standard barrel is a backbored, oversized tube that the slug can get wedged sideways in. It's NOT safe to fire slugs of any type through it. They may work a few times....but there will come a time when it won't and it won't be pretty.

You can, however, switch the 835 barrel out for a 590a1 5+1 shot (six shot, NOT a nine shot!) barrel and fire slugs through it.