Many .45-70 barrels are ported. Where ported barrels shine is with the slower powders which maintain a higher muzzle pressure. Brakes work by diverting gas in a direction not in line with the barrel, and the higher the muzzle pressure and the more the gas (heavier powder charges) the better the brakes work. The downside is that the heavier powder charges give higher recoil in the first place.
With the 300-grain bullets (which I believe make the best bullets for most handguns) the same powders that give highest velocities in longer barrels will generally give the highest velocities in the short barrels. I find powders like H4198, RL-7, AA2015 and H322 give better performance in the short barrels, both in power and accuracy. They also give rather high recoil in the light Contenders. An exception is BlueDot, which gives me the best accuracy in my 16" Contender with pleasant recoil. Velocities are not real high, but they are high enough to kill deer at 100 yards easily. I have had poor results with jacketed bullets and the faster powders like H4227, but others have different experiences.
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