Oh and Guest:

Your "source" is actually describing what is properly known as a "combination gun" or "swivel breech" gun- the paradox gun was a breech-loading weapon invented by George Vincent Fosbery, V.C. and patented by the British firm of Holland & Holland in 1885.

It was basically a double (heavy) shotgun with the barrels rifled for several inches at the muzzle so that it could be used with solid projectiles with reasonable accuracy. The rifling also functioned as a alchoke for use with shot.

The term "drilling" ordinarily applies to a breech-loading combination gun using fixed ammunition.

Last correction I'm going to make, Amos. ;>)