Tom - in my opinion, .54 cal. is at the upper edge for a 15/16" barrel - using a patent breech of course. Even at that size, I would hold my max loads to about 120gr. 2F GOEX.(which is suitable for any game we have in NA)
I have owned 3 .58's in the past that would not shoot accurately with less than 140gr. 2F GOEX. The larger bores do develope much lower pressures - but - recoil comes into it with light barrels = light guns.