Bolt thrust?

[M4arc]

Can someone please explain "bolt thrust" to me?

Maybe I'm behind the curve but I've never heard of this or experienced this...that I know of.

I'm no slouch at troubleshooting but this one is new to me.

[K.L. Davis]

The term is being misused on some forums as of late...

Bolt thrust is the amount of pressure that is generated against the bolt, it is calculated by the maximum pressure and the amount of surface that pressure has to work on -- in two cartridges with equal pressure, the one with the larger "base" diameter (internal case dia) will have more bolt thrust.

Bolt thrust must be considered when making a gun, to make sure that the lug shear is strong enough to support the cartridge.

[M4arc]

The term is being misused on some forums as of late...

Bolt thrust is the amount of pressure that is generated against the bolt, it is calculated by the maximum pressure and the amount of surface that pressure has to work on -- in two cartridges with equal pressure, the one with the larger "base" diameter (internal case dia) will have more bolt thrust.

Bolt thrust must be considered when making a gun, to make sure that the lug shear is strong enough to support the cartridge.

Okay, that I understand. But what does it have to do with extraction? The context that I saw this being referenced revolved around ripping rims off of cases and to me, that doesn't make any sense. Maybe that's what you mean when you say it's being misused?

Thanks for the explanation

[Robb Jensen]

Okay, that I understand. But what does it have to do with extraction? The context that I saw this being referenced revolved around ripping rims off of cases and to me, that doesn't make any sense. Maybe that's what you mean when you say it's being misused?

Thanks for the explanation

Especially since the bolt is pulled by the carrier from the barrel extension, not pushed rearward by the casing.

[M4arc]

Ah, okay! That makes sense.

[K.L. Davis]

Gas system, pistons, ergo grips, heavy buffers, super springs, battery compartments, telescoping stocks, spare parts, yards of rail space, folding things, QD this, snap lock that...

The single biggest "flaw" of the rifle design (imho) is that it does not have initial extraction -- and for the accesories, the best thing I have ever seen was a real cleaning kit.

Designing a firearm from scratch is not really a simple task... oh sure, a lot of us have made what is essentially a crude zip gun and it works, but getting the timing and pressure and manufacturing clearances and field clearances and all of that is not really easy.

The bolt is pushed back by the cartridge... that is why it is locked in place. If you have ever seen a rifle fire out of battery you can appriciate this! But, if timed properly, this stops as the action unlocks and ripped case heads almost have me looking at timing issues first and foremost.

Cartridge cases are designed with a taper, so that after a brief extraction, the case is essentially free in the action -- this is where initial extraction really pays off... after that, the extractor really is little more than a counter point for the ejecter to work the case against.

It is possible to have too much taper... as it is not enough.

What you are probably hearing some folks talk about is "bolt velocity"... but this is easy to counter -- at issue again is that several designs on the market today compromise proper action timing and in way too many cases, the people doing the design work dont really understand what is wrong.

Today, computer models make things a lot easier, but I know of exactly one firearm that went from CAD to production with no design testing and improvement. With the AR "mods o'plenty" that are out there, many folks are forced to live within the physical envelope of the design and COTS parts from whatever they can find for modifications.

In answer to your question... if the cartridge case design is sound and the chamber properly designed and cut, ripped case heads would lead me to first think the the action is unlocking while chamber pressures are moderatley high, contrary to logic, when the pressures are high the case will usually not try to "squirt" out, but will be stuck quite solidly in the chamber...

If the bolt/extractor tug hard enough on the stuck case, the head will tear.

eta: the formula for bolt thrust is easy, it is the work surface multiplied by the max chamber PSI

AREA * MaxPSI

Area of course, is Pi times the radius squared -- or Pi * (internal dia/2)^2