Home Forums Hunting & Shooting Gunsmithing Bolt thrust and chamber finish

320 then 400 is as fine as a chamber should be. Finer is the same as lube, no grip.

That's what I've been told, and this barrel makes me agree. Have you had personal experience with a chamber that's too fine, and if so, what did it do?

If the chamber is plated or coated with a finish which provides some lubricity (not sure that's a real word), the case will certainly fail to grip the chamber walls, just as if it were oiled. I do not think this truly affects bolt thrust, however, unless deflection of the bolt is sufficient to stretch the brass.
For those who think a high polish is just the same as lube, make a sizing die of 416 stainless and polish it to a mirror finish inside. Shove a fired case into it and see how solidly it sticks. A polished surface is nothing like a lubricant.
If the rifle and ammo have some head clearance and the chamber is lubed, whether with oil or a coating, the case head will be deformed because it contacts the breech with some velocity. This is not increased bolt thrust but the deformation of the case head is the result of energy transfer due to the velocity at which the case moves back. GD

Once again, what does the chronograph say? If you are magically getting a free 200fps......

People state on here that there no bolt thust as the case grips the chamber wall. If that is the case why did TC never chamber there contender for cartridge like 243, 308, 7mm08, ect, they had to use a stronger encode frame. I know from experience when chambering a 700 action for 30-378 and 338-378 the action went out of head space after 200 to 300 rounds

I don't think anyone is saying that.

Oh ok then what are they saying you can't tell me that if you polish the chamber wall smooth as a piece of glass that is
t doesn't increase bolt thrust

I started the thread asking if anyone had first hand experience (not theories!) of increased bolt thrust with a polished or slick chamber. I don't think anyone was suggesting there could be a situation with no bolt thrust at all.

Got ya. I don't polish chambers not to cause bolt thrust

Sorry, I'm having trouble following the double negatives in your sentences; not sure what you mean by that last one.

Do you mean that you don't polish chambers, because you don't want to cause increased bolt thrust?

I don't polish

This may be the Experiment with chamber finish you refer to.

http://varmintal.com/a243z.htm

Very few perhaps no one may have experience with your exact circumstances. Polished Melonite chamber finish really reduces the number of folks with same experience. I would buy into the theory that the polished Melonite finish will simulate a lubricated bore. Friction coefficient changes significantly with the polished SBN treated steel as I understand the process.

Any Idea How much head space you are starting off with b4 fire forming the brass in question? Or are you experiencing the same flow issues with fire formed & neck sized brass.
I am no expert on the situation at hand yet I fear you really are pushing the pressure limits in this case . I will be interested in seeing what more is learned with this delima.

Stay Safe!

Thanks for the link.

Headspace was .002"-.004", although I also tried partial sizing (not bumping the shoulder) and crush fit on the fireformed brass.

The only thing that made the brass flow go away was firing the cases 5+ times at lower loads, then they would handle the manufacturers loads with no brass flow. When I sent it back to the manufacturer, he tested the barrel with his own brass (which had been fired multiple times too) and proclaimed it OK.

Seems to me the brass work hardening masked any issues with the barrel, but he disagreed.

Doesn't matter now, when I got the barrel back I found he'd "polished" the chamber and somehow left a sharp lip in the neck, cutting through the melonite on one side. Sent it back a second time and was finally able to get a refund.

I'll be making a new barrel for myself in another version of this same wildcat, but with the shoulder bumped forward a little. No melonite treatment or slick polished chamber this time though, we'll see how it works.