Re: Breech/Bolt thrust during fireforming

18 September 2004, 11:04

I'll reread Ackley berore I comment. My work withthe tape
was rather crude, but consistant. Ok Shooter hasn't been around for a while but I think he still has a page.
I got into this by shooting a 35 rem with a little headspace. If you increase loading there willbe a point where the primers will not bulge. The "40 KPSI" is a result of back calculation using the NRA pressure formulas.
I should think that once the base moves back static friction is out of the picture, and Pressure x Area is the force.
Good luck!

15 September 2004, 20:30

Clark

Chris,
Thanks for the kind words.
I can't seem to hurt guns with bolt thrust, with the exepion above, but I have split the chamber on a half dozen.
Maybe I could get that book written if I bought a hunting cabin and stayed there and shot everyday

Right now I am working long hours, 7 days a week doing other people's engineering.

15 September 2004, 23:25

Chris F

Clark,

Quote:

I can't seem to hurt guns with bolt thrust, with the exepion above, but I have split the chamber on a half dozen.

Do you believe your bolt face is absorbing the full effect of the bolt coming back and it's strength of the breeching that's showing, or do you feel that the case/chamber adhesion is mitigating a significant amount of the backthrust?

Also, would you happen to have some case or chamber dimensions for the 250/3000 cartridge so that I put myself through your 30/30 trigonometry exercise?

BTW, I just read through Varmint Al's site with the empirically derived Mu's and it doesn't appear that he directly tested lubricated brass on steel (or did I miss that somewhere?) He does however make an assertion that a greased case would have minimal effect without any experimental data that I can see. My only experience remotely relevant to this is having stuck a case (okay cases) in a die for lack of case lube. What a difference lubrication made there. Any thoughts or experiments there?

14 September 2004, 09:21

Ricochet

Quote:

I'm also wondering, can I get away with fireforming using factory ammo or would the cartridge base slam into the bolt face before the case could expand completely? Am I completely wrong?

Huh? Why would you think having the case head "slam into the bolt face" would stop the case from expanding? The case will expand in all directions.

This whole thing about cases gripping the chamber wall and reducing bolt thrust is silly nonsense. Yes, it happens. But, it's trivial and unpredictably variable. You can do calculations till the cows come home about the theoretical amount of thrust taken up by stretching the brass under the conditions where the effect is maximized, i.e., a case of minimum dimensions fired in a chamber of maximum dimensions with the case all the way forward in the chamber at the moment of firing. (That is to say, maximum headspace conditions.) That involves making some assumptions about the hardness of the brass and its frictional adherence to the chamber walls that actually are rather variable. And NONE of it occurs when you have a minimum headspace condition with the case starting right against the bolt face at firing, which actually is an ideal condition. It also is unlikely that you'll be firing a gun with unlubricated chamber walls as normal bore cleaning and oiling leaves the chamber oily, and few of us will spend the amount of work it'd take to throughly degrease a chamber wall before shooting. Gun designers aren't foolish enough to take some estimated figure for the thrust reduction due to brass stretch off of the thrust due to gas pressure in the head and design the gun to be safe with that thrust. No, they take the full thrust on the case head and add a generous safety factor to that. Any gun that's unsafe to shoot with lubricated cases and chamber walls transferring the full pressure to the bolt face is unsafe to fire, period. The many erroneous statements to the effect that it's unsafe to shoot with lubricated cases or chambers due to increased bolt thrust (often written by experienced, knowledgeable authors who should know better) probably came from Hatcher's determination that several accidents at the National Matches in the '20s resulted from deliberately greasing bullets on the firing line, the grease taking up the chamber clearance around the case necks with an incompressible substance and thereby causing higher bullet starting pressures and raising pressures generally. A light coat of oil that doesn't fill the space can't do this and is safe.

The "Ackley Improved" cartridges are overhyped, IMO. There's no way the small increase in case volume they have over the standard versions can produce the claimed ballistics without pressures being considerably elevated. They may indeed reduce brass flow in the front of the cases and reduce the need for neck trimming by gripping the chamber walls more tightly with their reduced taper, but the tradeoff for that is that the stretching (which has to occur to fill the headspace) will have to occur at the head end of the case, which results in case wall thinning and potential separation.

If you've rechambered a .30-30 to a .307, you're going to have to determine whether the rifle's strong enough to safely handle .307 pressures. You won't be able to reduce the bolt thrust enough by any change of chamber shape to make it safe if it's not. Your calculations should ignore brass stretching effects entirely.