generally the max gr for a 1/12" is in the 165-168 gr range, 1/10" is 173-175 for .308 bullets. there are a number of other factors involved also. the most important variable is bullet length. then probably bullet rpm out of the barrel. if you have longer barrel to get the bullet rpms up you can shoot a longer(ie...heavier) bullet in a slightly slower twist just fine. with your barrel only about 20" you don't get as much rpm from barrel length, so you have to match the bullet length/ weight more accurately. the "palma" bullet would be a good starting point, but IIRC they are fairly long for their grain weight (155 gr). try something with a lead core and/ or a flat base to get the bullet length shorter per grain weight. so, to finally answer your question the lighter weight bullets should give you better performance.