Yeah I was told about 15 years ago that 80 grains is max for 3f in a 54 cal. My one TC Renigade shoots great at 75 gr. of FFF Goex never has missfired I use 85 gr in both my 50 cals and want to try it in mi Pro hunter 209x50
With Swiss black powder...2fg is perfect for my 54 cal flinter. 3fg Swiss blows patches...a tad too hot. I have shot the 3fg Swiss out of my White Super 91 .504 cal. inline & know a lot of guys that use it with outstanding results.
I prefer Goex and Swiss 2f in my TC .50 flinters over the 3f because my rifle prefers it. I do use Goex 3f in my RMC to shoot sabots and it likes it. I need to test it out with the Swiss 2f and see how it does yet. I don't think there is a pressure issue using 3f in a 50/54 but I think you have to let your rifle tell you which is better
I think alot of people who use the 3f do so because they can use 1 granulation for charge and prime. Some of the experts I have talked to in the past claim that a flinter should use the 2f over the 3f for a more consistant ignition. I like the 2f with a patched round ball and the extra speed from 3f isn't worth much in power, range or trajectory.
Our family of flinters uses Goex FFFg in all our sparkers; .36, .45, .50, .54, and .58 calibers. We don't push the loads in any of these guns because they perform quite well with reduced charges of powder. More isn't always better with black powder.
Common sense. 3fg is a little faster and a little higher pressure than 2f. If you go pushing envelopes it could have disastrous effect. The old rule of thumb was 3f for anything 45 cal and under and 2f for 50 and over to perhaps 75 cal and over where folks suggested 1 fg
Modern steel and manufacturing produces much tougher steel and barrels. Modern Guns can usually handle a hotter powder within reason. Just don't go putting 150 grains of 3fg in a 10 guage shot gun with a twist barrel. You may actually get away with "super loads" in big bore guns, but then again, you might not. Why risk it.