A BSA Stutzen RB-2 in .25 cal for $50? What an awesome score - congrats !!!!
While .25 cal was one of the standard calibers that the gun was available in, they are indeed uncommon in this caliber.
They are a fine rifle - but - these rifles have one weakness that you should know about.
The piston head is either aluminum or nylon, fitted to a steel stub, under tension from a rubber "buffer" washer, installed between the piston head and the piston to absorb the impact of the piston at the end of it's stroke. Over time, this rubber washer deteriorates, and disintegrates, which will eventually gum up the gun, with reduced velocity and degraded consistency of shot to shot performance. Continued use with a disintegrated buffer will eventually hammer the piston head until it basically wedges itself solid into the bore - making for an exceptionally difficult repair at that point - especially with the aluminum piston head.
This is all that is typically left of the buffer in the average Airsporter or Mercury.
The piston all cleaned up, with new buffer and o-ring seal in place.
Well worth checking the gun out to ensure the longevity of the gun, and your enjoyment of it.