Agreed they are probably for a bolt.

But since they are rimless the R should not be in the head stamping surely!

I suspect that S&B have screwed up somehow and have either misstamped a J cartridge case with JR. or worse, they have misstamped an S cartridge meant for a bolt gun with JR.
Bullet diameter measurements would sort the latter out but as I don't intend to use them it will be sometime before I get around to it.

Thanks again

All the best

Skeeterbd