I'm about to put in for some variations and just want to confirm what I should put down for the actions of various types of firearm.
Muzzle loading black powder revolver
Muzzle loading black powder revolver converted to nitro
Single barrel muzzle loading pistol black powder (either flintlock or percussion cap)
muzzle loading rifle black powder (wither flintlock or percussion cap)
Bearing in mind that this is a 99% US-based forum there will no doubt be some head-scratching here wondering what you are asking about.
However, I live in Cambridge, England, so I can answer you.
1. You must mention the calibre of the muzzleloading revolver.
2. You must mention the calibre of the muzzleloading revolver - mentioning that it is a nitro conversion - you will NOT be allowed to convert it back to BP.
3. Calibre of muzzleloading pistol - it does not matter how many shots it fires.
4. Calibre of muzzleloading rifle - making certain that it IS a rifle you are asking about. A smooth-bore, providing the barrel is 24 inches long or more, is classed as a Section 2 shotgun.
I'm surprised that your club secretary has not advised you on this matter - that is part of their job as the your referee. Also, it's odd that you haven't asked any fellow club members - in our club we are always talking about the next bunch of guns we want.
It's a good idea to apply for your BP explosives license at the same time - this will make things easier at renewal time as both will get done together.
Be advised that you will need to include Triple 7 BP substitute on the explosives license - it seems that after many years the PTB have decided that it is actually an explosive, like BP. The other subs - Pyrodex P, RS and Select, only require you to show your FAC.