So I was wondering if you could use a muzzleloader with small shot (size 8-6 lead shot) for hunting grouse and quail during the end of the hunting season for them?
In Idaho, upland game birds (except forest grouse) may be taken with a firearm as long as the firearm is classified as a shotgun. This can be found on page 18 of the 2014-2015 Upland Game, Furbearer and Turkey regulations.
18 USC 921 states:
The term “shotgun” means a weapon designed or redesigned, made or remade, and intended to be fired from the shoulder and designed or redesigned and made or remade to use the energy of an explosive to fire through a smooth bore either a number of ball shot or a single projectile for each single pull of the trigger.
This means that only smooth bore shoulder fired firearms are legal for the taking of upland game birds (except forest grouse). If a firearms contains a rifled barrel, this firearm is not classified as a shotgun and would be illegal for the taking of upland game birds (except forest grouse) regardless of the projectile(s) fired.
A smooth bore muzzleloader would be legal in this instance.
The same is also true of handguns chambered in calibers such as .45 long colt. Although this firearm is capable of firing .410 shot shells, the barrel is rifled. Federal law requires all barrels under 18 inches in length to contain rifled bores. This firearm would not be classified as a shotgun and would be illegal for taking upland game birds (except forest grouse).