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Old September 10, 2012, 11:33 AM   #1
Sweet Shooter
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Bi-metal copper coat thickness.

It's the old "bi-metal projectile/barrel wear" discussion again.

So per another thread about not cleaning out the copper as we (some) used to... and that some of us still do feel the requirement to strip it out... I was at the range and noticed that one of the Herter's copper coating was scraped forward by the bullet seating process. I discarded it.

I've sanded and looked at a cross section of the steel jacket rounds and while there appears to be a substantial coating of the copper it dawned on me that the main baring surface—not ogive—has probably been scraped during bullet seating to some extent. Now I'm wondering again if that copper is in fact doing a darn thing to help the barrel out?

I'm shooting .311 bullets in 7.62 x 39 with a .311 bore. Most of the brands that I've tried have been .311 or .310, even the U.S. manufactured ones (I measured them). So I'm wondering if that copper coating amounts to .003 reducing my baring surface to .308 which is too small for this bore.

Also, would there be an issue with long term storage if the copper is rubbed off and the steel case neck is in contact with the mild steel jacket in places?

Thoughts anyone?
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Old September 13, 2012, 11:35 AM   #2
Sweet Shooter
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Aww... come on...
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Old September 13, 2012, 06:57 PM   #3
Dfariswheel
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The copper plating isn't there to "help the barrel out".
The plating is there primarily to prevent rust of the steel bullet, and a lesser benefit, to help lubricate the cartridge during the feed cycle.

Pretty much the instant the fired bullet contacts the rifling and is engraved by it, the plating is gone from the contact areas.

With the plating gone, whether it's in contact with the case or not, given the right conditions of storage the mild steel will rust.
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Old September 16, 2012, 12:11 PM   #4
sc928porsche
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The gliding material (jacket) is usually more than .005 thick, and most of them closer to .010. Most rifling is .003 to .004 deep. The "shaved" area of the bullet does not go into the core.
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Old September 16, 2012, 05:31 PM   #5
Dfariswheel
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The bi-metal 7.62x39 (AK-47) bullet doesn't have a copper jacket.
As with most of the Russian military and commercial ammo, the jacket is mild steel with a thin copper wash plating to prevent rust, over a lead core.
This copper coating is very thin and can be easily scraped off as the OP has indicated.

If you recover a 7.62x39 steel bullet at the range you'll find the copper plating is largely gone from the rifling and friction in the bore.
Only commercial type bullets in the 7.62x39 have a real copper jacket over a lead core, and most of those are American made bullets.
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