I am not an expert, i never been and i think i will never be as i think i learn something everyday.
My opinion/experience is, I had a Tikka 223 in 1:8" and it shot everything i put through, Hornady's 55gn soft points - 75gn "match", the worse groups was about 1.2" at 100mt or yd, the thing about 1:8" twist is that it will shoot heavier projectiles that will give better/less "wind drift" at long distances for been heavier.
My suggestion is, ask for another gunsmith opinion depending what he say you cut and re-crown or may just re-barrel the thing all together. Dramatic and maybe extreme but should work.
M
Bookmarks