Barrel Rifling Questions

[eagle-man]

So yesterday i got out a Mosin Nagant that my son's friend had some troubles with. He was saying that he could only shoot a 12" group at 50'. Not the best... So I was looking at it, and the barrel was horribly dirty. First time I had dry particulate matter come out of a barrel when I ran a brush thru. After scrubbing for a while, it was looking quite a bit better. I did notice that it seems like the rifling is cut rather deep on this barrel. Which got me thinking.. and so I am hoping someone may have some wisdom and knowledge that they would like to share.

Questions:
1. On the Mosin Nagant's, why did they cut the rifling so deep? Any special reason?
2. On modern rifles, how is the rifling cut?
3. On modern rifles, what is the general difference between the diameter of the rifling and the diameter of the grooves that are cut compared to the bullet for which the gun is built? Say for a .308" bullet in a 30-06, what is optimal for the diameter of the rifling and the grooves?

Thank you!

[John Taylor]

Groove depth changes with caliber. A .22 might have a depth of .002" while a .50 might have a depth of .005". Older military rifles most likely had deeper rifling with the idea that they would last longer and work with variety of bullets sizes. A modern .308 usually has a bore size of .300" and a groove size of .308" which give a rifling depth of .004". On your second question, rifling was first done with single point cutters. With mass war production a broach was used, then they went to button rifling. Now most of the big companies use hammer forge rifling. Button rifling is what most after market barrel companies use and the custom barrel makers use single point rifling, which is a true cut rifled barrel.

[I'll Make Mine]

My Mosin has pretty deep rifling; it slugs at .300 bore and .313 groove; it's not unheard of for the groove to run as high as .316 with the bore still at .300 to .301 (though they're usually closer to .303 with a groove that large).

The Mosin is an 1891 design for then-new smokeless powder (though the ammunition was reportedly issued with black powder, briefly); the rifling may have been cut deep because designers weren't yet sure how the small bore, fouling, and high velocity would interact (black powder repeaters tend to have deep grooves to keep them safe with fouling from many shots). Once the design was fixed, however, no changes were made other than things like length, bayonet and sights; the action parts interchange between an 1891 and an M44 (even the Chinese M59, produced up to 70 years after the original Mosin started production, are supposed to interchange with other Mosin models), and barrel specs were kept the same as well.

BTW, deep rifling doesn't seem to do anything bad for a Mosin; mine shoots at around 2 MOA with the original rear sight and Smith Sights adjustable front, and Brown Bear 203 grain soft point ammunition.

[nhrifle]

The Mosin I had was rifled similarly, but I did enjoy shooting it. Factory ammo didn't shoot as well as my cast loads did. I shot Lyman's 314299, and when I was having a better than average day I could hold a 6" to 8" group at 200 yds. I would say make sure the bore is as clean as you can get it and try a fat lead slug. Might surprize you.