Formula for bolt thrust?

[Harter66]

Has anyone used or have one to use. I know Mausers have been built from 22-250 to 458 WM and seem to be safe even full tilt. There must be some way to generate a usable number,to know that a change is safe.

[frankenfab]

Well, you can calculate the area of the case head in inches by using Pi times the radius (in inches) squared, then multiply times the PSI of the load.That gives you totak bolt thrust in lbs. Then, if you can measure the amount of lug contact you have, you can calculate the PSI of thrust on the locking lugs. That will give you a rough approximation, but there are other factors that reduce or increase this a bit, such as chamber finish (rougher reduces bolt thrust) or oil or lubricant on the case (increases). I believe shoulder angle and case body taper are also factors.

I am sure that someone more knowledgeable than I can provide a better explanation.

[Harter66]

I had thought along those lines too, I would think nominal/maximum bullet weight would enter at some point also

[NoZombies]

bullet weight effects PSI in the chamber, PSI and other factors, as mentioned above, effect bolt thrust.

I've never found a formula that was easily accessible that provided results based on anything close to all of these factors. Every gunsmith I know has his own "formula" and many of them disagree.

As far as how much thrust a gun will take, that's a whole other ball of wax. You have to take contact surface, steel type, heat-treat, and a slew of other factors into account.

Most people just overbuild.

[frankenfab]

I am sure headspace is a factor as well, since excessive headspace would allow the case head to gain velocity before contacting the bolt face, thus increasing the energy transferred by a minute amount.

On the few guns I have chambered, I shoot for 0 headspace when cutting, and polish to get 0005". I know I am a believer in the theory (or possibly proven fact?) that lapping the locking lugs increases the strength of the action.

[lurch]

My thoughts on the matter:

From a pure perfect world physics approach, the area of the case head multiplied by the maximum chamber pressure will give the maximum bolt thrust that can be obtained. Other factors such as chamber finish and oil, which both affect amount of gripe the case has on the chamber walls during firing, thickness and yield strength of the brass at whatever point where the grip on the chamber is less than the stretching forces on the case, and probably the phase of the moon will only server to reduce the actual bolt thrust. If the case head separates anytime before peak pressure then the bolt will see the full thrust of the head area multiplied by the chamber pressure.

That said, you have to take into account how much of an overload you want to handle and adjust the pressure used accordingly. Lug contact area, where the lugs actually contact the receiver in relation to the bolt body, how sharp the corner is between the lugs and the bolt body, etc. all affect the stress seen by the bolt. It would take a fairly good attempt at an accurate 3D solid model and intimate knowledge of the exact material and hardness of the bolt to get a truly accurate estimation of the stress and to know exactly how far below yield stress any given application is.

The point being, there are a lot of variables that are not known exactly and are taken into account with varying degrees of accurate (or inaccurate) approximation when trying to come up with what the actual bolt thrust is. It's no wonder there are a lot of folks that disagree on the subject. Design with adequate margins (over build) and all should be well.

[BeeMan]

Here are a couple pages from one of the better sites on the web (if not the best) for answering this type of question:

http://www.varmintal.com/abolt.htm
http://www.varmintal.com/amod7.htm

Look around for more information.

FYI, I've done the manual hoop stress and other calculations, as well as reverse engineered a few barrel and action dimensions using static stress equations. It will make you think twice about pushing the envelope for operating pressures of firearms. Metal fatigue is not to be ignored.

BeeMan

[S.R.Custom]

As mentioned previously, worst case bolt thrust can be anticipated as maximum chamber pressure applied to the bolt face as a function of the contact area of the case head against the bolt. And in the case of rebated rim cartridges, figure the area of contact using chamber diameter, not rim diameter.

And... never assume a metal or it's heat treating. While it's convenient to say that Type 'X' Mauser Actions are good for a 'Y' amount of bolt thrust, keep in mind that heat treating of an individual weapon may vary with the intended/original chambering.

In other words... When you're fiddling about with cartridge conversions, pressures & bolt thrusts should never exceed those of the actual proof pressures of the original chambering. Anything beyond that is uninformed speculation.

[jerry_from_ct]

It will make you think twice about pushing the envelope for operating pressures of firearms. Metal fatigue is not to be ignored.

BeeMan

That reminds me of discussions of and trying to discourage the conversion of straight pull M95's, that became available for awhile.

Just because an action may meet technical criteria, the above quote is well worth heeding.

[jerry_from_ct]

In other words... When you're fiddling about with cartridge conversions, pressures & bolt thrusts should never exceed those of the actual proof pressures of the original chambering. Anything beyond that is uninformed speculation.

Well Said................................

[Harter66]

Thanks BeeMan,
Although I can absorb only half of that I will reread them several times.

I may have led the kind folks astray a bit as this was a learning question rather than a specific application question.

[jerry_from_ct]

Thanks BeeMan,
I may have led the kind folks astray a bit as this was a learning question rather than a specific application question.

Is there a difference....

[Harter66]

I'm trying to sort out questions for myself , real stumpers ,like if an action will handle 900gns at 15kpsi and 1500 fps w/.7 sq.in.of head, why can't it be loaded w/.5 sq.in.at 18kpsi w/450gn at 1800 fps. E.g. 12ga to 45-70 knocking on the limits door,as opposed to throwing a 460 S&W in a 410 chassis,instant scrap!

[MtGun44]

Relatively straightforward calculation for the basic thrust inside the case, but this is
not the full bolt thrust. The case carries some of the thrust load.

Internal area of case head times the max internal pressure in PSI, is the start. If you
do not know this as an actual PSI value instead of CUP or LUP, which are NOT actual
PSI, then you can only approximate. I believe (but am uncertain) that the CUP and
LUP values will always be lower than actual peak pounds per square inch pressure.

Subtract the case wall cross section area times the yield strength of the brass - if you
assume that the case wall will stick to the chamber wall perfectly. 3 key assumptions
that will introduce error. One - WHERE does the "sticky part" end, exactly? and Two, if
we know (or estimate) this location, what is the wall thickness there? Three, what is
the yield strength of the brass at that location? Not easy because the brass is fairly
soft at the neck, fairly hard at the head and some value in some unknown distribution
in between.

We can put an upper bound on the amount that the case can hold, since the
maximum yield stres for fully hard cartridge brass is known, and we can section a case
and get wall thickness just ahead of the head. The actual wall thickness is probably a bit
farther forward ( look at where case separations occur) and probably a bit softer
than fully hard.

We can get a fairly good approximation by the above methods if you have actual psi
peak pressures, and make some reasonable estimates for the rest.

Bill