My experience is different to the powders recommended so far.
Since I got my first 338, the ONLY powder that gave great accuracy has been RE19, the best part of that is the fact it runs 100fps faster than any other powder I tried behind the 225gr Accubond. I ran with a load in either Win or Rem brass, WLRM primer and 76gr of RE19 with 225gr Accubonds, this gave me 2890fps in a 26” barrel and 2820fps in a 24” barrel.
With 200gr bullets and lighter, 2 powders stood out for me, Win 760 and H4350.
Picking a powder is not governed by barrel length, it is governed by what performance you want.
For me, I look at several manuals, starting with the bullet manufacturers data, then look at several other sources and look for the powder that provides the HIGHEST velocity across all those sources, then I evaluate IF it is going to be sourcable, as Alliant powders here are impossible to get, then I will select the three TOP powders across those manuals that are the highest velocity providers. The reason for this is because those powders are the most suitable in efficiency for that expansion ratio. Then I will load 3 cases starting at 5% reduction from max and work up in 1gr increments and shoot groups, choose the tightest group and then do a ladder at 300mtr with +/- .3gr increments.
I'd start with H4350
With my 26" barrel, I've had good luck with both H4831 and H4350. With shorter barrels it preferable to favor the faster powder, but no need to go crazy far in that direction. Just stay off the crazy slow for caliber side of the burn chart and you will do fine. The slowest powders for caliber produce the best velocities in longer barrels.
This statement about faster powders being preferrable in short barrels is complete BS.
The powder that provides the FASTEST velocity in a LONG barrel will also be the powder that provides the fastest velocities in a SHORT barrel.
It is time for this MYTH to be buried forever, it is simply false and misleading.
