I thought I responded to your 7/57 question but it's not showing. Since you were kind enough to give me a tip on the 45 Colt loading, here is what I said:
Check the twist rate of your barrel. If it's much slower than the standard (9.5 I think)it may not shoot heavier bullets well.
Or go to a lighter bullet that has the terminal performance you apparently are seeking in a heavier bullet. I'm shooting 140gr Barnes X Triple Shock in my 7/08 (very like your 7/57), using 43.5 grains of Varget. Velocity is over 2950, very accurate with great penetration (I have literally had one penetrate the LENGTH of a large buck, with very little loss in total weight...one of the petals broke off).
But you have to learn how to load them, and use data meant of Barnes bullet. Suggest you check out their web site.

Have been loading long guns for some time but am relatively new to handguns. Here's my problem: Shooting a 4&5/8" Ruger Black Hawk in 45 Colt, I can get several loads to group with 200 and 250gr RN cast and 250 jacketed bullets to pretty much the same point of aim.
Started playing with some 255gr Shooters Choice (lubed cast, flat point,w/o gas check). Using 9.5gr of Unique (the same powder that was used with the other cast bullets)and 9.5 gr TightGroup,crimped, both loads shoot distinctly to the left. The rear sight is already one or two clicks right for the jacketed hunt load; I'd have to move it WAY right to hit point of aim with the 255gr loads.
Here's the ?: I know from experience I can move the bullet up or down somewhat by varyinbg the powder charge. Does the same apply to get horizonal movement? If so, should I be decreasing the charge? Can't go up much,10.1gr is listed max with Unique. Or, should I try another powder?

I had kinda the same thing, only a larger difference in bullet weight. I had mine all set up for 250 gr cast bullets, and was shooting very well. I switched to 300 gr XTPs, and the point of impact shifted 12" to the left (at 20 yds, no less). Nice groups, though. The only things I could come up with was either a different hand position that allowed the gun to torque more - or the increased bullet weight casued more torque that 'overpowered' my normal grip to a certain degree. It was weird though. I ende dup dropping to the 250 XTPs and the POI shifted right back over and I was able to make a minor sight adjustment to get them nicely centered.

Only 5 grs of bullet weight shouldn't have too big an effect, unless the bullet shape is much different - more bearing surface maybe? You could try dropping velocity a bit and see if there is an effect. I tried shooting with a cheap pair of gel bicycle gloves and it did seem to help me be more consistent during practice sessions. I've had better luck using Lil'gun than small charges of the faster pistol powders. That's a big case, and filling it up more will give you less velocity variations. maxman

"the point of impact shifted 12" to the left (at 20 yds, no less)"
That's exacty how mine is behaving. I'll try backing off a bit but one of my objectives is to maintain velocty up around 1100 fps to possibly hunt with the cast bullets.
I use Lil Gun with the jacketed XTP's but got started on the Unique and TightGroup after reading some gun mag. There's my problem!!!
Thanks Max.

Try another powder. As a rule, varying your MV will not make a horizontal change in POI. At least not a PREDICTABLE ONE.