Is it 3fg? if so it should only be used in very light loads in a 58.
Max loads forth also depend on the manufacturer of the gun and the strength and quality of the breech and barrel steel.
I read posts on the net about using 200 grain loads of super whup butt powder with $4.00 plastic tipped bullets in guns that aren't intended for loads even a quarter of what is being suggested. 40 years ago there was some real muzzle loading junk sold to the public. Some of it isn't fit for 50 grains of black powder and a patched round ball.
You didn't say what make of gun. Go by what the manufacturer suggests. Most manufacturers have loads listed on line, but keep in mind that triple 7 is about 15% hotter in pressure than comparable black powder. Max loads are nearly worthless and usually are far from the most accurate. It is a "Do not exceed" guide.
Keep in mind that 54 and over caliber guns do not follow the same pressure curves as smaller caliber guns. Even with round balls, very high pressures occur rather quickly as charges are boosted up wards.
The stuff is very hard to ignite in a flintlock anyway, if you have a flinter, you may just want to sell it or give it away.
Assuming you have a modern made 58, and percussion, I still would not exceed 60 grains with a round ball and just use it for plinking. Yes I am being over cautious. I have seen guns that blew up with mild target loads because they were short started. Not pretty.
I use Triple 7 in my TC inline 58 caliber and measure by weight not volume and have zero problems with it..I find the lighter loads like 80-100 grains offer best accuracy.3F would be a little more pressure then 2F but it should not damage the gun..TC makes quality steel and I own many TC rifles with zero problems.I rarely use round balls anymore but use sabotted bullets which are heavier..
Most of the substitutes are made to be measured by volume equal to black powder loads. Measuring by weight could prove disastrous for a powder that is lighter than black powder. For instance a volume for volume black powder and pyrodex are very similar, but Pyrodex weighs only about 75% per unit of volume as black powder. Measuring actual 100 grains of Pyrodex would be closer to the pressure of 125 grains of black powder. Since trip 7 is already about 15% "hotter" weighing out charges to equal weight of black powder could prove very dangerous. (I just don't know how trip 7 compares in weight to black powder, lighter, heavier or the same) I know when pyrodex first came out, Hogdon was very clear about using it volume for volume the same as black powder and warned against weight measurement to avoid overloading problems.
I'll have to dig out some powders and weigh the differences.
54 flint 1. Is your green mountain barrel 1 inch ACF? 15/16ths? 1.125 inches? that determines barrel wall thickness. Often the weakest point of the breech is the barrel wall at the point where the threads meet the barrel bore. Worse yet, I have heard ugly rumors about GM using helicoils to keep the threads tight. (It is a short cut method to tighten sloppy threads.) As I recall, trip 7 comes in different granulations like black powder does. Using 3fg trip 7 would develop significantly more pressure than your usual 2fg black, unless you reduce charges accordingly. That's why I suggested a very low plinking charge of 50 or 60 grains.
54Flint,I weigh my charges but the powder is sold to be measured by volume..I have had no problem doing it this way..I find the lighters loads are the most accurate..I just always measured by powder scale..I do not own any powder measures by volume..I have never blew up a gun..