Dan,
I haven't worked at 50Y - yet, but I know at 25M my Super Barrel puts 5 consecutive shots into a hole that leaves the ink showing on the 9 ring of a WRABF target, for 3 consecutive groups. Yup, those JSB's seem to be what it likes, or the other way around, no doubt about it! And the best part is, I've got 5 tins (2500 pellets) each of 2 different Lots that shoot like this, and 2 more Lots that are damn close. I haven't tested the rest of the multiple Lots of JSB's I have yet, but for now I'm not going to worry about them.
Dave