The cylinders strength specs has nothing to do with pressure.! The guns cannot take the forces generated if a heavy bullet is driven to over 1,000fps for very many shots.
The cylinders come with velocity spec of 1000fps because (SAMMI) pressure can easily be held by the cylinders. I believe the cylinders (kirst 5 shot) are tested to 35K.
Case Head Thrust...is what will break the old C&B's. And the simple fact that the bullet going down the barrel is trying to pull your revolver in half. Case Head Thrust is velocity and bullet weight....NOT pressure. This is part of the reason that Lead cast bullets are suggested....This is still a Case Head Thrust issue because it is harder to compress copper into the forcing cone.
The 45acp runs just fine at 21,000+PSI out of a conversion safely and the cylinders are made for the 45acp..but it only reaches 800 or 900fps+/- with a 230gr bullet. ..If you run a big cased 45colt to 21,000+ you can and will easily exceed 1000fps, even with a big 300gr bullet. And that will tear your revolver apart!
So while the ACP runs just fine 21k, the 45c will damage your gun at this pressure...Because of Velocity.
The conversion cylinders are short so they limit the size of bullet you can really use. One of my personal favorite loads is the .451 XTP 230HP at about 800fps out of an old 5 shot Kirst ...yeah it's jacketed, but the .451 diameter keeps things safe. I have over 1,000 of them down the tube and my Remington is still shooting just fine.
But truthfully the load I shoot most out of my 45Colt conversion is a little 160gr BigLube EPPUG bullet over 15gr 2fOldeEynsford with a 38special case of cream of wheat in the case.....This load shoots GREAT and hits spot on with my cap&ball cylinders...and I don't need earplugs to shoot a rabbit or coyote when i'm out riding.