Anyone have this handy?
jeffe

Stole this off of Lilja's website, there is a LOT of information in this article of his:

http://www.riflebarrels.com/articles/custom_actions/bolt_lug_strength.htm

Bolt thrust is easy to calculate. Only two inputs are required. They are peak chamber pressure in PSI and as mentioned, the inside area of the case head that the gas pressure can work on. The formula then is:

THRUST=AREA*CPSI Where:

AREA=3.1416*(HS/2)^2

HS=the diameter of the inside of the case head.

I sectioned some cases and measured the inside diameters and found that they were as follows:

CARTRIDGE CASE INSIDE DIAMETER (HS)
222 .300"
PPC .370"
308 .385"
MAGNUM .420"
378 WBY MAG .500"
50 BMG .680"

The thrust is measured in pounds per square inch.

Chamber pressure (psi) times the inside cross-sectional area of the cartridge case (sq inches) = total or maximum bolt thrust in pounds force. Bolt thrust can never be higher than this, you can quibble about the amount the thrust is reduced due to case "grip" to the chamber if you want.
Pounds force divided by the contact area of the bolt lugs (square inches) = compression force applied to the lugs/seats in PSI

Actually, I'd be curious about how much the case's grip on the chamber wall helps to alleviate bolt thrust. If the case/chamber is lubricated, does bolt thrust increast by 1%, 10%, or what?

Jeffe,

As a moderator, you should know better than baiting the trolls!

Regards,

Pete

<hides bucket of bait>

Awwww Pete

quote:

Originally posted by MarkL:
Actually, I'd be curious about how much the case's grip on the chamber wall helps to alleviate bolt thrust. If the case/chamber is lubricated, does bolt thrust increast by 1%, 10%, or what?

The above calculations assume 0 grip, IOW the amount of thrust you'd see if the case/chamber were oiled. Always base your safety factors on the worst case seneraio, you'll live longer that way.

And PeteE, that's the first time someones suggested I'm a troll, should I be happy or offended?

quote:

Originally posted by Tailgunner:
Pounds force divided by the contact area of the bolt lugs (square inches) = compression force applied to the lugs/seats in PSI

I think the bolt thrust would be a "shear" force to the lugs and compression to the seats, but I'm no engineer. FWIW Tailgunner, I don't think you're a "troll".

Tailgunner,

I was not suggesting that either you or Fireball168 are trolls, but believe me now the "chum" is in the water they will come!

Regards,

Pete

With a dry case and chamber the case will "stick" until around 40,000 psi.
Then it comes back with the same force as it it had beem oiled. That is why primers will protrude with excessive headspace in lower pressure cartridges. In higher pressure cartridges the primer will push back, then the case comes back making the primer look like a rivet. Finally it is the primer firing that pushes the case forward, not the firing pin.
Good luck!

If you don't want to section cases to measure inside diameter, a simple formula to use
which incorporates the amount of thrust the
case holds and the difference between the OD
and ID is this one----Base area times peak
pressure times 2/3 gives a slightly high safe result to work with, for example a big Weatherby case, .582 base at 60k
gives 10,600 lbs thrust with that formula.
To compare to formula with inside measurements
and subtracting what the brass holds using inside measurement of big WEA case
of .500 you get 12,000 lbs of thrust from formula and subtracting the 2400 lbs of thrust the brass holds you have 9,600 lbs thrust.Ed.