I recently bought my 300S and put new seals in. As my local shop was out of R10 rifle, I tried R10 pistol and they were very consistent. I then got some R10 rifle and although needed to adjust sights slightly for difference, the consistency is still excellent, so in mine I have left it with R10 rifle but it would be just as good on R10 pistol.
I have not tested other pellets as these in my 300S are far more consistent than my wobbly aim and I have managed a 58/60 score by wobbling on to the target at the right time.
Enjoy shooting your 300's whatever pellet you choose - from what I have read, they will not be far off.