Is a 1-10 twist ok for stablizing the 300 gr sierra mk in 338

Yes the 10 twist will work just fine.....

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I HUNT LONG RANGE.AND YES I USE MATCHKING'S.

They shoot great in my 340 and it is a 10

Dog

is a 24" barrel ok for the 338 ultramag or is a 26" or longer needed?

I would recommend a 26" or longer..The 338 RUM will not show what it can do unless a longer barrel is used...If I were to build one it would be 28 or 30".My 300 Rum wear's a 30" tube...So 26 as a min,28 or 30 would be better....

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I HUNT LONG RANGE.AND YES I USE MATCHKING'S.

Boyd Heaton;concerning the .338 RUM,30"barrel;which is the best one I can buy.

A Border Barrel. We have tried a few of their 1:10 in 338 (Lapua Mag) among other calibers (and barrels) for BR, and: Yes! They are good, scertainly better than the available bullets.

http://www.border-barrels.com/

AKI