Re: rebarrel for 338 Lapus Mag
.338LM is along action, as was pointed out. Some companies make a "magnum" or "XL" action for the magnum loads like the Lapua. Remington just uses their standards long action. If you want a great deal, look into the new Remington 700P MLR. It's a .338LM with detachable box mag and comes with a brake. It sells for $1248 MSRP, but LE/MIL price is $972. It has a 26" barrel and 1:10 twist.
If you want to have a custom barrel put on, know that all .338LM barrels need to have a 1:10 twist. It's also recommended to have a barrel that is 26" or longer.
If you want a custom build, look into a custom company like GA Precision (GAP). Otherwise, look around for custom gunsmiths that specialize in precision rifles. Some gunsmiths include RW Snyder Gunsmithing or Short Action Customs.
There are lots of custom actions out there like the Surgeon XL, GAP Templar,and my personal favorite, Badger Ordnance M2008 (going on my .338LM build). All these are great, but the action costs more than the whole Remmy 700P MLR.
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