Normally moderate loads of 3fg are fine. BUT, and that can be a really big BUT, A few years ago, there were a couple hundred target barrels made that were really light weight with very thin barrel wall thickness. Some 50 cal barrels were made in 7/8 inch across the flats and a few in in 13/16 inch across the flats. These were for offhand shooters who would be shooting with really light loads of 2fg. Some of these barrels were used to make regular looking long rifles and half stock hawken style guns. Any heavy load of 3fg in such barrels will necessitate plastic surgery.
As a general rule, most production guns are safe to withstand moderate loads of 3fg, however, there are some really unsafe exceptions out there. Also, this is only so long as you are shooting patched round balls. Shooting a conical or minnie ball completely changes the pressures the gun is subjected to.
When I got my TC Hawken in 50 cal with 15/16 inch barrel, many years ago, Chuck Dixon of Dixon's Muzzleloader shop told my that in my TC, up to 80 grains of 3fg would be fine with a patched round ball. While I am sure that a small amount extra won't low up that gun, there are alot of poor quality imports still floating around out there. One of my rifles in the early days was a Markham Arms 45 cal Hawken, with a very thin barrel. Made in Bulgaria or some weird place. The lock wore out within 200 shots and it did say in the Manual not to exceed 45 grains of 3fg powder. Got about the $60.00 I paid for it along with an education. There were some even worse quality 69 cal smoothbore flintlocks sold in stores around Philadelphia in 1978. Those things were terrible and several blew up. I worked at a Sporting goods store and we were told by the distributor to "break/cut them in half and throw them in the dumpster." I know some of them were never returned to the store and are still out there somewhere.
