Too many possibilities... first make sure it's not the shooter...no being critical, the 338 kicks, so pad your shoulder get everything the rifle is resting on as consistent as possible including your shooting position and how you hold the gun..be consistent and conscious of it. Shoot the group have someone load the chamber while your back is turned...is the chamber empty or does it contain a live round...you don't know. Have them watch to see if you flinch. Believe me I was shooting the 50BMG and the rifle always shoots better than I was shooting, had to stop and rethink, analyze, and concentrate on shooting, groups shrunk back to normal. Next is scope, bases, rings etc tight to manufacturer's specs. Inspect the bore and chamber area with a borescope. The load ...maybe the barrel doesn't like the bullet, the powder, the velocity it's being shot at. If you reload seating depth can change things to the positive. Try a few different bullet weights, & types copper, match, cup & core check for improvement. Bedding,.. anything on or in the stock contacting the barrel down to the action. If you hand load try some 200 JSP gr bullets with imr 4831 and imr 4350 in moderate loads up to book max... if ya go to heavier bullets...RL 22 is many time accurate also.
But I'd work with the lighter bullets to break in the barrel and the shooter. By the way I don't "break in" barrels, shoot clean, shoot clean, method, anymore, quit many years ago. I inspect them with a bore scope, check to see that the bore is indeed hand lapped ...then polish the whole bore with Iosso paste especially in the throat and lighter to very light as I get to the muzzle. Any tighting of the bore dia will be at the muzzle, Done, new barrel go shoot it a hundred rds...don't clean til I get home. Maybe something here you can use, but I'll tell ya we have been getting some inferior products out of some supposedly quality manufacturers the last few years...if your gun has an accuracy guarantee and it still doesn't shoot ...see if you can send it back.