Groove depth?

[andym79]

Hi guys I was looking a bore groove chart and most barrels have a groove of .006 and .008 over bore. Meaning a groove depth of .003-.004 pretty standard.

I did however notice an exception 318/330.

My question is two fold:

1 what difference does groove depth make when shooting cast?

2 why is the 318wr 12thts different?

Thanks

[Larry Gibson]

The Mauser 7.9x57 started the deeper grooves with a .314 bore a .323 groove. That was +/- of corse but the groove depth was inteded to be .005 to .006. The concpt was the deeper grooves would give longer barrel life given the corrosive nature of the ammunition back then.

[tazman]

1 what difference does groove depth make when shooting cast?

In my experience, the deeper the groove depth, the better it grips the boolit and the less driving band shearing there is.
I had barrels in a Taurus PT92 and one in a Springfield Armory P9(40S&W) that had very shallow rifling. Cast shot very poorly in either barrel even with very hard lead although jacketed shot fine.
In the case of the Taurus, I installed a Beretta 92 barrel which had noticeably deeper rifling and the problem ended instantly. The P9 went down the road since there wasn't a similar option available at the time.
Larry has already addressed the other question.

[andym79]

Thanks guys so deep grooves are good for wear but may also help with cast?

[Larry Gibson]

Definitely

[andym79]

Hi Larry is that a definite to both or just barrel wear?

Does that extra .002 help or is it just about sizing over grove?

I ask because I am building a rifle and am trying to establish if a 330/318 has any advantage of the more common 338/330 the only real difference being groove depth.

[Larry Gibson]

Both......it's why a milsurp 8x57 with excellent bore can be extremely accurate with quality jacket bullets and cast at higher than the RPM Threshold with the right design cast bullet.

[andym79]

What would be a good bullet design, a loverin? Or am I on completely the wrong path.