I picked up a Browning A Bolt in .338 Win Mag. Used gun.
I've gone through my normal load development process: case prep of fired cases, trimming, flash hole deburr, etc..
I've got the OAL to lands, but, unfortunately, the magazine on this rifle won't let me get within .1".
Anyway, I tried to work up a decent load with H4350 and H4831. H4350 is the better choice, and gives better velocity for load density.
I'm using the 225 gr. Accubond, which is the bullet weight I envisioned when I bought this rifle.
I'm getting decent results around 70 gr. H4350. When I'm working on a load I generally load and shoot 3 round groups in various loadings to get some idea of where the sweet spot might be.
Here's the problem: every other group throws a flyer. Almost always horizontal, left or right. Turns it into a 3-4" group.
When I got the gun, I bedded the action. Tuned the trigger. Worked the stock to free float the barrel.

FYI, it's not me. I shoot Palma as a High Master.
Barrel looks good, went through a long cleaning process. Throat could be eroded, but most used hunting rifles I've bought don't seem to have been shot much. Particularly in .338. This could be the exception. Crown looks good. 26" barrel.

I'm considering trying a 250 gr bullet. Don't know the twist, but it should stabilize the lighter bullets best. I'm not interested in other powders. I've got a big stash of H4350.

Ideas of people who've worked with this caliber would be appreciated. I'm looking for consistent 1.5" grouping, 2600 FPS would satisfy me.

My “standard” load for the 338 is a 250 grain bullet with 64.0 grains of WW760 powder. It’s been working for 25 years.

Is the flyer the first round chambered by hand?

Nope. All mag fed, and flyer has no pattern.
Also, rifle is cool for all shots. Generally these sessions of maybe 30 rounds take two hours. I bring my handgun and run drills while the rifle cools between shots, or work with another rifle on a separate bench.
This puzzles me because the flyer is SO far out. Only thing I can think is bad throat erosion and, like I said, most used hunting rifles I've seen haven't been shot much. I have a friend with a bore scope over in Idaho. I'll bring it over there when I get a chance and post results.

I've heard that many folks like H414, which is the same as W760.
I'd like to experiment, but I live in the middle of nowhere. Buying small lots of powder shipped is expensive with the hazmat.
And I've got a LOT of H4350, which is use for the .260 Rem I shoot in competition, and the 7mm Rem Mag.

Have you tried varying the seating depth?

Can't, except to make it shorter.
Generally I use a comparator and fired, modified case to establish seating depth: find where the bullet touches the lands, and I generally start by setting it back .020".
This works great for competition rifles like my Palma gun- Barnard single shot action.
But this is a hunting rifle which feeds from the mag, and that limits how close to the rifling I can get. In this case I can't get closer than .10", unless I modify the magazine.
I may make up some rounds set .020" off the lands and try them just to see how they shoot.