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You would better off with plan A for serious subsonic work a 1 in 8" twist barrel is best the Remington's 1 in 12" twist won't handle the 200 grain plus projectiles if you are going that route

Save your money and buy a decent barrel I like Remingtons but I wouldn't use one of their barrels to rebarrel anything check out True-Flite their basic hunting barrels are better than Remington...