You could pick up one of these inexpensive folding 12 gauge shotguns, shorten the barrel to 16", and mount sights on it.
And use it with an 8" barrel insert.
Unfortunately, the company is skittish about offering .357 magnum chamber inserts, but does offer a .45 Colt one, as well as a .38 Special one.
With a 250 grain JSP and 17 grains of 2400 from an 8" barrel you would reach about 1000 FPS.
This load would certainly be safe in any modern Colt single action clone in .45 Colt, so it should be fine in this shotgun, and equals the velocity from a 20" barrel carbine with the mild factory loaded cartridge. So, at 50 yards it would drop a deer.
You could also use it with 12 gauge slugs and bird shot, making it all the more versatile.
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