I don't know what the Illinois law is going to say. But if it's turns out like the Louisiana and Mississippi "primitive weapon" law it will cover the type of gun and cartridge, not the propellant. The gun must be a single shot exposed hammer gun designed prior to 1900. The cartridge may contain smokeless powder.
Here's some poop on Louisiana's rules.
All of the approved primitive weapons meet the criteria set forth by the Louisiana Wildlife and Fisheries Commission as authorized primitive firearms. The criteria is a single shot, breech loading, metallic cartridge rifle with metallic cartridges loaded with either black powder or modern smokeless powder, .38 caliber or larger, of a kind or type manufactured prior to 1900 and replicas, and reproductions or reintroductions of that type rifle having an exposed hammer. The pre-1900 distinction applies only to the firearm and not the ammunition. All approved primitive firearms may be fitted with magnified scopes.
Approved single shot breech loading primitive weapons:
Sharps rifles or replicas
Remington Rollingblock rifles or replicas
Ballard rifles
Maynard rifles or carbines
Burnside carbines
Frank Wesson rifles
Farrow rifles
Remington Hepburn rifles
M1873-1888 Springfield (Trapdoor) rifles and carbines and replicas
Snider (British) rifles and replicas
Wesson & Harrington 1871 rifles
New England Firearms or Harrington & Richardson Handi rifles in caliber larger than .38
Winchester M1885 Hi Wall or Lo Wall rifles or replicas (Also Browning B78 or 1885) .38 or larger
Knight KP-1 in caliber .38 or larger
CVA Optima Elite in caliber .38 or larger
Traditions Pursuit break-open single shot in .38 caliber or larger
Non-approved single shot breech loading rifles:
Ruger Number 1 and Number 3 (Reason: No exposed hammer)
Thompson Center Contender or Encore Carbines (Reason: Designed after 1900)
Mossberg SSi Single Shot Rifle (Reason: No exposed hammer and designed after 1900)