From Jamie on Utreon:
"I know naval gatling guns like the 20mm CIWS uses gain twist rifling, but are or were there any commercial or military small arms that used gain twist rifling?"

Yes, there have been some significant uses of progressive (gain) twist rifling on military and civilian small arms. These include:

Colt percussion revolvers (1848, 1849, 1851, 1860, 1861 models)
Custom barrels for Sharps, Stevens, and other rifles by Harry Pope
Carcano Model 1891 rifles and carbines
S&W .460 XVR revolvers

There are certainly others, but those are some of the most significant.