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Xlucine

How do primers even work?

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Or more specifically, why don't they burst? You have huge pressures behind them, they're thin and not really solidly fixed in place, and yet they are expected to stay intact - how?

 

Me and colli were discussing this on TS yesterday, and there's no way inertia of the firing pin could be providing meaningful support - from squinting at a pressure Vs time graph of a 30.06, and assuming the firing pin can be modelled as a piston of radius ~1mm acted on by the chamber pressure, the applied impulse is on the order of kgms-1. So that firing in should be moving backwards at a considerable velocity, given the light weight, and over the ~1millisecond of the firing process it should move several mm back - yet primers retain the impression of the firing pin after firing. The peak force applied was on the order of kN, so I'd be very surprised if the firing pin spring was providing enough of a force to resist this

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Primers don't have very much explosive in them. I've popped new unseated primers before; an M80 creates a much, much bigger explosion (cue videos made by dumb kids) The primer cup itself is enough to contain the primer's explosion.. The pressure produced by the powder charge is successfully contained by the bolt resting against the primer.

Why don't they burst? They do; popped primers are one of the major signs of overpressure (though if you're popping primers, you need to back way the hell off, as even the lower end of unsafe pressures usually doesn't pop primers). Read this for a little more info.

 

Why do they retain the firing pin impression? Good question, I'm not sure, but can think of a few possible reasons. One is that on the other side of the primer cup is the anvil, which may prevent so much pressure from pushing back against the indentation left by the firing pin. Another is that primers are usually pretty hard, and they have a yield strength above the pressure being produced. Keep in mind, the pressure being produced is high, but the area's very low, so the force involve is also probably low (assuming 60,000 PSI and 1mm diameter, that means 3.27 Newtons force are being exerted on the primer. This is equivalent to the force of gravity on something that weighs 2.4 ounces). A third theory is that there's enough pressure escaping around the primer to equalize the pressure on it. I think this is probably likely.

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We're trying to figure out why the primer doesn't back out of the case under pressure.

 

At peak pressure there's metric fuckloads of force on the primer, the same primer which was installed from the back and is basically only press-fit into the pocket.  The firing pin doesn't seem massive enough to stop it.

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We're trying to figure out why the primer doesn't back out of the case under pressure.

 

At peak pressure there's metric fuckloads of force on the primer, the same primer which was installed from the back and is basically only press-fit into the pocket.  The firing pin doesn't seem massive enough to stop it.

Oh, silly, you know why. 1. There's fuckloads of pressure against the sidewalls of the primer causing it to grip the primer pocket. 2. The bolt is there stopping it.

If you look at cases "before and after", you can see that the primer actually does expand. Not only does it get more "square" around the edges, it also protrudes further in a fired case compared to an unfired one. In fact, most of the time you can actually just make out machining marks imprinted on the face of a fired primer, left over from when it was trying its damnedest to become one with the bolt fact. I'm looking at a fired .30-06 case right now that exhibits these markings.

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Keep in mind, the pressure being produced is high, but the area's very low, so the force involve is also probably low (assuming 60,000 PSI and 1mm diameter, that means 3.27 Newtons force are being exerted on the primer. This is equivalent to the force of gravity on something that weighs 2.4 ounces). A third theory is that there's enough pressure escaping around the primer to equalize the pressure on it. I think this is probably likely.

 

Erm, I think you made a mistake in that calculation - 60000 psi over that radius comes out around 300N, not 3N

http://www.wolframalpha.com/input/?i=60000psi+*+%281mm%2F2%29^2+*+pi

That's pretty significant for such a thin sheet of brass. You do have a good point about the bolt keeping the rest of it in - I'd forgotten it was so close around the firing pin. As for the pressure release, it sounds odd to me - the path from the propellant to the primer is quite open, so you ought to end up with a substantial amount of gas blowing past the primer if it is going to equalise the pressure. Also, with that gas cancelling out the chamber pressure, you shouldn't expect to see machining marks on the primer surface

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Oh, silly, you know why. 1. There's fuckloads of pressure against the sidewalls of the primer causing it to grip the primer pocket. 2. The bolt is there stopping it.

If you look at cases "before and after", you can see that the primer actually does expand. Not only does it get more "square" around the edges, it also protrudes further in a fired case compared to an unfired one. In fact, most of the time you can actually just make out machining marks imprinted on the face of a fired primer, left over from when it was trying its damnedest to become one with the bolt fact. I'm looking at a fired .30-06 case right now that exhibits these markings.

 

That doesn't make sense.  If there is enough friction between the primer and the primer pocket to prevent it from exiting the pocket intact (which I doubt), then it would become a question of whether the primer's own sidewalls have enough tensile strength to prevent the ass-end of the primer from detatching under the force, which seems incredibly unlikely considering that cartridge case heads do not.

 

Is the firing pin hole just enough smaller, and the forces transient enough that it doesn't make it all the way?

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Erm, I think you made a mistake in that calculation - 60000 psi over that radius comes out around 300N, not 3N

http://www.wolframalpha.com/input/?i=60000psi+*+%281mm%2F2%29^2+*+pi

That's pretty significant for such a thin sheet of brass. You do have a good point about the bolt keeping the rest of it in - I'd forgotten it was so close around the firing pin. As for the pressure release, it sounds odd to me - the path from the propellant to the primer is quite open, so you ought to end up with a substantial amount of gas blowing past the primer if it is going to equalise the pressure. Also, with that gas cancelling out the chamber pressure, you shouldn't expect to see machining marks on the primer surface

 

Oops. I think somewhere along the lines I converted mm to cm wrong.

Anyway, yes the bolt is right there keeping it in. Both of you, stop trying to prove reality wrong!  :P

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Primers can back out if the headspace is excessive.  When you see primers that look like they are melted, it is generally a case of the primer backing out, starting to expand radially, and then the case catching up and smashing it flat against the breechface.

 

Don't forget that some designs were actually built around the concept of using the rearward movement of the primer to actuate the bolt.

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I recall that Garand had some primer-actuated rifle prototypes, and these were ditched because the Army was going to move to staked primers that would not reliably actuate the action.

 

I guess there was that weird SPIW design with the piston primer as well.

 

Can't think of any other primer-actuated designs.

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Most of the AAI SPIW models used primer piston actuation.  The 9x51mm spotting rifle for the LAW80 and SMAW uses a variation on the scheme crossed with the high/low pressure concept.  Roughly speaking, a .22 Hornet cartridge is inset into the base of a .358 Winchester case.  The setback of the Hornet case upon ignition actuates the bolt.

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