British made parker hale barrels have progressive depth rifling the muzzle is tighter than the rest of the bore. the minnie has to be small enough to go into the muzzle then when fired will expand into the rifling and be squeezed down as it travels down the bore iirc correctly they may also be gain twist also. If you cant get it to shoot with proper sized minnies than you mave a skirt stuck in the breech area or the nipple might need to replaced. also see the other tips for getting it to shoot i posted above.
hitler, stalin and mao were progressives in their time