221FB = efficient and accurate
I purchased one last year - Sako A1 with Hart barrel. Falling in luv with this caliber was quick and painless.
I originally had a Cooper ordered in this caliber but the order got misplaced
I would seriously consider its twin - the 20 Vartarg - supposed to be even better than the 221 for accuracy - would love to get one in a Cooper - Clay @ Prophet River has a couple in stock - very tempting

Regarding bullets - most accurate was 40 gr. Nosler, 35 and 40 gr. V-maxes are accurate as well, with the 35 gr'ers producing the higher carnage factor on prairie gophers.
Longest kills on a small town of rats - 285 yrds

All my loading was with Lil Gun - 16 gr. was stretching the primer pocket - reduced to 15.5 grains producing .5-.75 five shot groups if I do my part.
After doing more research, I just purchased some Reloader 7 - supposed to be more accurate with the velocities close to a 223?? Too early to tell as I have only shot a few rounds with this powder - accuracy seems promising though

I will say, it is hard to beat how well the Lil Gun powder meters.