Go past around 60- 75 rounds with plastic-cased shotgun buckshot or slug ammunition, and you'll find that the barrel can become hot enough to melt the plastic cases. It becomes quite possible to chamber a round, then have it either stick in the chamber, or leave sufficient plastic residue to prevent the chambering of a followup round.
There are a few things you can do to help matters, to include leaving the chamber open between shots if things go beyond a dozen or so rounds, to get all the cooling possible. And of course the composition of the plastic used in differing brands of ammunition varies, so some can be expected to do better than others, and your particular gun may have a tighter or looser chamber than others.
Check out what some of the trap/skeeo shooters who fire 200-round strings use- but remember that their lighter target loads don't generate the power, recoil, heat or pressure of full-house buckshot and slug loadings.
Good Advice.
You just convinced me to carry two S.G.s.