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I just finished slugging the barrel on my WesterField 30-30. I wasn't for sure if it was a Micro-Groove barrel or not but after Sharp's suggestion that if it has lots of grooves then it likely is. From the best I can tell it has 12 grooves in the barrel. That should mean that it is in fact a Micro-Groove. After numerous checks/measurements with my micrometer (not my caliper), it appears that the groove diameter is .309". My cast bullets are dropping from the mold at .310". In my thinking these bullets should be good, (at least in this barrel), as-is with no further sizing needed. Am I correct in this thinking? (Please be patient with me as I'm new to casting and slugging barrels). I forgot to mention that these bullets are gas checked. If I'm good then I can go ahead and work up some test loads and head for the range. Any comments are greatly appreciated.
 

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Sounds good to me accident! You're right on the money. The only thing I ever heard about Micro-Groove barrels and cast bullets is they don't like to be driven over 1600 fps.

Edit: Thought just occurred to me. I've always had the best results in the 30-30 with cast bullets with one of three powders; IMR-3031, Reloder 7 and Win. 748. No doubt others work well, those are simply the ones I've used most and liked the best.
 

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I second the Re7. Back when my eyes worked I’d get sub moa groups with iron sights out of a Win 94 using that and a 173g cast Lyman 311041 I believe was the mound number.
 

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Not much to add except +1 with Sharps and Firpo. Your .310 bullets are good to go as cast. RX7 is my go to powder for cast rifle loads. My 30-30 mold also drops bullets at .310. I use a .310 sizing die to set the gas checks without sizing the bullet down. My 30-30 barrels run .308 or .309, so I'm .001 or .002 above groove diameter. Lyman's manual cautions against velocity above 1600 fps in micro groove barrels, I have a Marlin 336 in 35 Remington with a micro groove barrel that will do close to a minute of angle with air cooled wheel weights at a bit above 1450 fps; shoots patterns rather than groups at much above 1600. My two cents.
 
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