What caliber are you looking at, Fernie. Reason I ask is, I have a couple of 410/45 LCs on SB1 frames. I got a 45 LC carbine barrel, and got a good lock-up on one of the SB1s, so I have been using that combo with no problems what so ever. Granted, I am keeping my loads down in the 8 gr of unique range, so the pressures are probably in the 14 K range or less, but black powder in that caliber would be even less pressure than that.
My reasoning was, that even with the long free bore of the 410/45LC, nothing in the instructions said a person couldn't shoot hot loads through that gun. Some of the hotter factory loads can generate considerably higher pressures than 14 K, and possibly over 20k, so I figured the SBI receiver should defintely be up to the task. Also, nowhere on the frame did it say SB2 like on all of the other rifle barrels. All it says is 45 Colt Classic Carbine.
I know we're not talking apples to apples if you're looking at a 357 mag or 44 mag. But, a 45 colt would definitely be a good BP option.