Actually you can build the 81mm in black powder form. However you must make the bore small enough that a mortar round will not fit app 77 mm is 3 inches. You must also weld the basecup in place so that it cannot screw off. If you follow these two its a go. There was a company that was building them for a while. contact bill at www.bigshotsofidaho.com. he can help ya with it.....
That's how I understand it as well - as long as the ID is smaller (ie 77mm vs 81mm), welded cup with no firing pin...its considererd muzzel loader. Basically, its the same thing as 37 grenade luauncher vs the real-deal DD 40mm grenade launcher.
Being new to this forum, I hope this is the correct place to post regarding to what I beleive to be a BP question. With 19 tax stamps and counting, I have a fair amount of knowledge regarding NFA (DD's, MG's, AOL's, etc..).
Thanks for all of the posts and info!
PBR