I would think yes in all firearms season except the primitive f/lock season.It would be interesting to know if you could use these in s/gun only areas.That could be a gray area but I would think they shouldn,t be legal there.
100 years ago Harry Pope made centerfire case ignited muzzle loader target guns similar to this. The primed case full of powder was inserted in the breech and the bullet loaded from the muzzle through a false muzzle. I asked about the lagality and was told no. The difference was that popes guns could also be shot with fixed cartidges but he guaranteed tighter groups when muzzle loaded.