I would recomend using a chronagraph to test the load SDs and ESs to start with. If they are
not under 15 ft/sec then you need to change loads. (Bullets ,powder,primer)
Mine did not like the 300 gr bullets or the H1000 so I switched to the 250gr bullets and Retumbo
and H4831sc and groups size dropped to less than 1/2 MOA and SDs below 10.
It is not like the 338s to shoot poorly unless fed the wrong ammo.
There are many things that can be causing your problem and you have to address them one
at a time so you will know where the problem is. all to often we make a lot of changes and
if it fixes the problem but we don't know what it was. So change things One at a time.
I hope this will help because I know how frustrating it can be when a rifle just won't Shoot.
J E CUSTOM