IIRC the .45 caliber Huntsman had a 1:56 twist. You can easily check this with a tight fitting patch on the jag...push the tight patch to the bottom of the bore...tape the ramrod at the muzzle and mark the rod in line with your sight on the tape...slowly pull the ramrod out, leting it turn with the rifling...if it makes 1/2 of a revolution, measure from the tape back to the muzzle and multiply by 2 to get your twist rate. You would need to travel 28" in 1/2 of a revolution for the 1:56. If you cannot make 1/2 of a revolution in the barrel, stop at 1/4 of a revolution. Measure from the tape back to the muzzle and multiply this measurement by 4 to get your twist rate
Example: If after 1/4 of a revolution your rod has moved 14", 14 X 4 = 56, 1:56 twist.
If after 1/4 of a revolution your rod moved 7", 7 x 4 = 28, OR after 1/2 of a revolution your rod moved 14", 14 x 2 = 28, both would be 1:28 twist.
I take it that the one you bought has a screw-in breech plug instead of the push in plug? I'm not real sure the .25 ACP will work in them, I haven't heard of anyone doing that. The old huntsmans have a flat firing pin for #11 caps, they will set off 209 primers, not sure how well they would do on the small rifle/pistol primers.
Let us know what you find out.