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For .30 cal rifles, the bore (land) diameter is .300" and the groove diameter is .308". That is a groove depth of .004" and we use .308" bullets to match the groove diameter. But, on my .223 rifle, the bullets are .224" in diameter. That is only .004" over nominal bore diameter (.22). So are the grooves in a .223 Rem rifle only .002" deep? What about microgroove barrels? Is there less groove depth in a microgroove barrel? If so, I assume a .35 Rem would have a land diameter of greater than .350" and still maintain the groove diameter of .358". I'd just go check it myself but I don't have any gage pins in that size range.
 

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If the specs are followed by the manufacturer the 35 Remington will have a bore diameter at .349 and groove diameter of .357 inch. The 223 Remington is .219 bore and .224 groove if cut properly.

A microgroove barrel as you know does have shallower grooves but more grooves than standard cut barrels which allows it play the same fiddle. Though I don't own or shoot a 35 Remington I'm told when shooting cast, a long bore riding cast is required for good performance.
 

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Discussion Starter · #3 ·
Okay. Interesting. Thanks for the info. I didn't realize the groove diameter is nominally smaller than the bullet diameter.


So, then the .35 Rem has a nominal groove of .004" per side and a .223 Rem has only .0025" groove depth per side.

I would almost think that a higher velocity cartridge would need more groove engagement with the bullet to keep from shearing the sides of the bullet off; especially with the tight twist of some .223 Rem barrels. With a 1:7 twist and muzzle velocity of 3200 fps, the rifling in the barrel needs to get the bullet spinning from 0 to 329,000 RPM in about a half a millisecond.

Conversely, I suppose with the larger diameter bullet, even though the bullet isn't accelerated to such a high RPM, the greater mass farther from the axis of rotation may require even greater force on the periphery of the bullet.

Probably need a physicist or engineer to calculate whether it takes more force to spin up a large diameter bullet to lower RPM or a smaller diameter bullet to a higher RPM. Heck... there's probably a calculator on the intardwebs that would make all the calculations if a body was inclined to search for it.

I just wonder why the depth of the rifling is what it is for various calibers and cartridges.
 
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