Ahall, would you be able to post the relevant formula if you have it handy. I assume the pressure is in c.u.p and the force is in ft/lbs ?
It would be a handy one to have on the wall to make the shop look more professional. LOL!
I have seen people post the formula for bolt thrust a thousand times JBCGUNS. I have asked a lot of them to explain it or do it in long math or even use it and it usually ends with a lot of erps and harumphs.
The first thing you have to understand is that the case is a vessel and it is also a piston. You can't use the outside of the case or the rim for an exact calculation. You have to use the inside of the vessel at its widest point. If you wanted the exact diameter you can section the case and measure the ID at its widest point. In most cases we just want close numbers for comparison which is just fine so what most people do is just measure the outside of the case at its base. NOT THE RIM.
The formula is normally given as radius squared times Pi times chamber pressure
For an example we can use the 30-06. The outside of the base of the 30-06, or its diameter is .471 inch.
Take the diameter and divide it by 2 to get the radius. Example .471/2=.2355
Now find the square of the area by multiplying the radius by itself
Example .2355*.2355=.05546025
Take that number and multiply it by Pi which is 3.14159
Example .05546025*3.14159=.1742333667975
Multiply that number by your chamber pressure which we will say is 55,000 PSI
Example .1742333667975*55000=9582.83
Which gives us 9,582 foot pounds of bolt thrust. Again this is rough and to high because we are not using the exact widest ID of the case. But its close enough for comparison use. Just going from memory, bolt thrust on most -06 cases is around 6,500 foot pounds if you use the measurement of the widest internal diameter of the case to make your calculation. The math is exactly the same only the starting number changes.
And now you know.
ADD NOTE: There is sort of an old rule of thumb for estimating inside case diameter. Its not exact either but it should give you a closer to reality measurement than using the outside of the base as a lot of people do. In most high intensity metallic cartridges the case wall thickness is roughly .045 inch thick, give or take. If you take .045 inch and double it and then subtract that from the base diameter it gets very close to the inside diameter of the case at the widest point.
Example for our 30-06. .045*2=.090 The 30-06 diameter of .471-.090=.381
If I do the math using .381 inch as my starting diameter instead of .471 inch it works out to 6,270 foot pounds per square inch, which is much closer to the actual bolt thrust.