It has the fast 1 in 10 twist apparently. This twist rate is meant to handle the heaviest of bullets. The 1 in 10 twist generally prefers 168 match bullets and up. Then again this isn't a .300 mag and you only have a 20 inch barrel, so overstablization shouldn't be a huge problem. It may be comparable to a 24 inch barrel with a 1 in 11 twist. You can try some 150 fmjs and 165 grain hunting bullets to see if they group. It should favor 168s and 175-178 grain match bullets most. It may even preform well with 190 match bullets. The 155 match bullets may definitely be worth a try as well. Stick to the ones with the higher ballistic coefficients (sierra palma match, hornady eld-match, and sierra tipped match kings).

Randy, I don't think Ruger specifies an optimal bullet weight. Actual long distance shooters tend to favor the 140gr tipped bullets. 123gr through 147 grains will work just fine. The heavier bullets are favored for better wind bucking at long range.