Most of the work I've seen on the gains from a standard 4 inch combat barrel to an 18 inch rifle barrel is, almost invariably, around a 2:1 power difference. I should buy a chrono myself, but from my shooting of magnum rounds out my Winchester 94ae, all I can say is the difference is night and day. The same 158 grain jacketed semi wadcutters, American Eagle, that I've shot as a budget round, would cut nice holes out of concrete blocks out of my 6 1/2 inch Rossi, and the rifle would turn them into an impressive pile of broken concrete and dust. Every animal I've killed with my Rossi would drop in its tracks, but every small game animal I shot with the rifle almost blew up, skinning squirrels, blowing up racoons, doing high power rifle grade damage. Even without a chronograph, I can tell there is a world of difference in that sealed chamber, and with the longer barrel.
With all the talk about powders, what about bullets? The OP noted hard cast bullets, which will perform better from a naturally tighter barrel, but will perform much worse in a slightly looser barrel. Its been noted that too hard a cast can lead to the bullet not expanding into grooves correctly, with resulting gas leakage, and subsequent loss of pressure. Indeed, it has been noted that harder bullets do not always lead to less leading, but instead sometimes more, via the poor seal's escaping gas burning off lead from the sides into the barrel.
Perhaps if the shooter were to do the same exact loads, but with many castings of the same exact bullet of different hardness, the results could easily flip around, and could be very informative, and surprising. Perhaps a softer bullet would seal better in the 28 and outdo the Colt, perhaps not. But it is a very important factor.
This thread has made excellent points that every gun is different on a small level, enough to throw off the idea of universally equal results from the same barrel lengths of firearm models. In order to figure out the "perfect" load for your individual revolver, one must not only experiment with various loads, but if one casts, the perfect hardness and lubrication to fit the exact nature of that particular firearm.
eally.
