i"ve heard that the 58 gr will break apart before hitting the target if you load them at 4000 fps mark.
the nice thing about shooting the heavier bullets, is better performance down range, but they are way more expensive.
at least the varmint grenades are cheap.
i'm not convinced you'l get the performance you'll want out of this gun past 300 or so yards. i've shot quite a few prarie dogs with mine and mine loses too much velocity to keep the bullet stable past 300. it definately starts to destabilize right around that point, then 400 is pretty much a 1 foot group at best from a bench vise.