SL, I also think he's speaking of AO rather than any BDC.

FG - if the scope lookssimilar on the objective endto the one pictured in this link http://www.swfa.com/pc-2085-198-bushnell-4-12x40-trophy-air-gun-scope.aspx- it's AO you are describing.

AO will not adjust your bullet's impact except to help reduce paralax as SL has indicated. Most scopes have no parallax adjustment - they are instead set to be parallax free at a given range (usually 100 yards). I can see very little use for an AO on a scope used on a muzzleloader intended for normal hunting conditions.