Alliant HERCO is, for all reasonable intents and purposes, a slightly slower-burning UNIQUE. At equal pressures in any rifle barrel length, I would expect slightly higher velocities from Herco. That said, it isn't so slow-burning that it won't burn completely in a 6" barrel (I've never gotten 2400 to burn completely in barrels less than 7 1/2"). Unburned powder from short barrels makes for lots of flash & noise at the muzzle, but not much else. Blue Dot or AA#7 might make for a decent compromise between the two barrel lengths.