I would think that from a 15 in. barrel you would get abt the same velosity from the .257 Roberts with alot less muzzle blast, if your set on .25 cal. I`ve got a 15 in. barrel .357 max. on an Encore frame and have it zeroed at 150 yrds., point blank to abt 175 yrds. I`ve taken 4 deer with it in the last 3 yrs., but in Oh. we have to use straight walled cases.
One hit for you, put a high, 25 to 27 in. by-pod on it. It`s just right for sitting on the ground and resting the grip on your knee, abt like shooting from a bench.