Each barrel is different from all other barrels. .338 barrels don't foul up any faster than barrels on similar velocity cartridges. You must be getting some copper fouling build up. Are you occasionally using a good copper remover like Sweets 7.62, or Butch's Bore Shine?

The issue may also be just barrel heat if you are not letting it cool enough between shots & groups. Glad to hear that your rig is shooting better.