I took a Lee mold that cast at .379 and lapped it to .381 It shoots great now.
Lapping = There are several differant ways to do it.
I cast a bullit and drilled a hole in it. Put an ez out in it. Coated bullit with lapping compound put drill on ez out. Hold handles together with bullit in the mold and drill. Do this a while- cast and measure. go some more cast and measure. Keep doing this till you get to the size you want.
Hope this helps