kamilaroi has a point. The .25/06 will give .257 Roberts IMP ballistics or close enough no animal will be able to tell the difference. Brass is cheap - range pickup being the primary sourse if you don't want to buy it. Anything on the '06 case will do and the FL dies will form them - seater die first, then FL die and ream or outside turn if necessary - seldom is.

I prefer the AckImp .257 myself - anyone and everyone has or can buy a .25/06.

Now, on a good bolt gun or single shot like the Ruger, both using a 24" to 26" tube, the standard Roberts will just about make 3,200fps with 100gr. and about 2,950fps with 120's. Any 57mm brass is good to go for either. I've made mine from 6mm rem, 7x57 and 8x57 brass along with .257 Roberts factory brass.

You merely have to decide if you want another 150fps to 200fps and how important that is to you.