Howdy
This question gets asked so often there should be a sticky about it.
No matter how well designed, no matter how strong the steel, you still cannot get around the basic fact that a revolver with a two piece frame will never be as strong as a revolver made of the same material with a one piece frame.
Period!
With a two piece frame, such as a Top Break, the impulse of recoil will be applied differently to one piece than to the other. Generally speaking, with a Top Break, the impulse of recoil slams the lower part of the frame, where the recoil shield is, straight back. Then, the barrel section, which is firmly latched to the lower frame section, plays follow up and recoils back too. Over time this stretches the top strap of the barrel section. I have seen this many times.
That is all there is to it. The more powerful the cartridge, the quicker this will happen.
Yes, somebody may be making a Top Break chambered fro 357 Magnum, but I guarantee you that if you shoot it long enough with full power loads, eventually the frame will stretch.
I bought this Smith and Wesson 44 Double Action Top Break about 15 years ago or so. This is a big, belt pistol sized Top Break, chambered for the 44 Russian cartridge. The 44 Russian was the predecessor of the 44 Special, which in turn was the predecessor of the 44 Magnum. Each new cartridge was more powerful than its predecessor. The 44 Russian is not a tremendously powerful cartridge, the recoil is quite mild compared to the 44 Magnum. Still, over the years the frame had stretched exactly as I described. Yes, it was not made with modern steel, but many, many mild 44 Russian loads managed to stretch it. When I bought it the latch had a little bit of play because the Top Strap had stretched over time. The play was not terrible or I would not have bought it. Still, when I latched closed there was enough play that the barrel could rotate up and down a few degrees.
Luckily I knew a crackerjack gunsmith at the time who was very familiar with the problems with Top Break revolvers. He drilled a pair of holes in the legs of the latch, and press fit some hardened steel pins into the holes. When he was done pressing the pins in place, they stood a little bit proud of the surface of the legs seen here. Then he very carefully filed down the surface of the pins until the latch slid over them and locked everything up tight. What he had done was make up for the amount the top strap had stretched over the years. I don't shoot this old revolver very often, and when I do I only shoot my Black Powder 44 Russian reloads in it. But if I were to shoot it a bazilion times, the top strap would stretch some more.
Pretty much the same solution Bob Wright mentioned above, but instead of building up the metal by brazing some more on, my friend solved the problem in a different way.