Dan's post addresses the original post, Bramble - not your 'converted' ctg. gun.
It is obvious the gun you are using & have 'built' and the manner it's being used is quite safe. I find it an interesting 'dodge' to the regulations - quite inventive of you.
Unfortunately, someone may now try Trail Boss in his cap 'n ball gun.

I've used Trail Boss in my .45/60 Sharps with excellent results at 100 meters off the bags. 15gr. fills the case to the base of a 390gr. bullets seated out 2 bands. It is indeed a bulky, light-weight powder and in pistol cases is loaded very close to, if not bulk for bulk. I would NEVER attempt to use it in a muzzlelaoding rifle or pistol. While the powder is light, in a rifle case such as noted above, it is developing over 25,000PSI. The load is light feeling in the 12 pound rifle, the report is light and the velocity is low, but for the speed arrived at ie: 1,200fps, the pressure is very HIGH - way too high for a muzzleloading rifle's system.