Couple of bits of "twist" trivia that Sniper did not cover in that very good post-
First, the term "gain twist". Rarely encountered now, it was used in early Colt black powder revolvers, and in the Italian Carcano rifle. The rifling starts at the breech end almost straight, and the rate of twist increases as you move towards the muzzle. Allows the gun to "spin up" a bullet as it accelerates. The Carcano used an unusually long bullet that would tend to "strip" thru standard twist rifling.
Second, the term "Paradox". These were incredibly large black powder cartridge guns that wee known as 8 bore or 10 bore guns (think 8 guage or 10 gauge) Yes, they were freaking cannon- intended to knock an elephant down. At the breech end, the barrel was a smoothbore. Partway down the barrel, it became rifled. That is why they were called the Paradox.