Interesting note on that - my old M17 never really liked 150 grainers but shot the snot out of the 180s. You may want to check the twist on your barrel - I don't think it was set up for the 150-165 gn slugs - the barrel on my M17 wasn't. And didn't that 03A3 come with a left hand twist? I think my M17 had one of those and I believe that favors the heavy bullet. My M17 was under MOA at 100m however, if you can shoot 3/8" at 60 yds, that would give you about 3/4" at 100m. Too big fer ya???
Tellya what - send that old 03A3 over here to me and send all your ammo with it and I will wring that baby out fer ya, yes I will. Mikey.