One might reasonably ask the same question of the NEF/H&R .410/.45LC combo gun and other, similar firearms without being too far off-topic. Looking at the open breech, one can see that the chamber walls are quite thick, but the fact that this is also nominally a shotgun leads me to wonder.
The chamber has little to do with the ability to handle the pressure. In a break open single shot, the locking mechanism is the most important item. The ability of the barrel lock and release to hold it closed is the limiting factor. A phone call or email would get you the answer as to what pressures it's rated at.
Don't expect the Rossi or any other 45/.410 combo gun to be accurate when shooting 45 colt ammo. The cylinder of the Rossi is able to take the 3" .410 shell. That means the 45 colt has an enormously long freebore,(smooth unrifeled area), before it encounters the throat and the beginning of the rifling. This means it is sliding, and accelerating. It hits the rifling going far too fast, stripping the rifling, maybe not spinning the bullet at all. The best idea is to limit the velocity and use the fastest powder you can find.
The 45/.410 is a compromise gun. It does neither too well, but it does both well enough to be useful.