Home Forums Hunting & Shooting Ask The Gunwriters Blowing Primers - Pressure Levels

Following on from the gas venting thread, I thought I'd better start another.

How much pressure is required to blow a primer? SAAMI list 65 000psi as the safe max pressure for cartridges, but when a primer does blow what was the pressure to do it? 70 000, 75 000psi?

I was wondering how much leeway/safety margin lies between 65 000psi and blowing a primer.

On another forum, a bloke gets 3 400fps with a 100gn bullet in a .257 Roberts and over 3 000fps from a 140gn in a 7x57. He has never reported any issues like blowing primers. If he's exceeding 65 000psi, how much more pressure till something gives?

I thought one of the gun writers might have seen pressure data indicating when primers will blow.

Interesting.

My experience is that pressures up to 70,000 or even 75,000 PSI often don't show conventional pressure signs--though that obviously depends on the brass and primers.

I have only managed to blow primers a couple times over the decades. Once was when my balance scale was inadvertently moved to a charge around 10% higher than the charge I'd worked up to. A general rule is that a charge of single-based rifle powder (and this was a single-based powder) results in approximately twice that much increase in pressure. So if my load averaged 65,000 PSI, the overload would average around
80,000 PSI.

I shot it on an average summer day, and the only sign of higher pressure was the chronograph reading, which was about 10% higher than normal. This is about what single-based powders does when the charge is changed: Velocity follows the percent of increase or decrease. Of course, I thought the chronograph was being screwy, and since there were no obvious "pressure signs" otherwise, I shot another round. About the same chronograph reading, and a little harder extraction. (The rifle was a Ruger No. 1.)

On the third shot the primer blew, probably because the barrel has warmed up. At that point I finally suspected the chronograph wasn't screwy, but I had somehow screwed up. Took the rest of the rounds home, pulled the bullets, and weighed the charges, finding the 5-grain "overcharge."

MD, just curious if you checked for extractor marks on the case head of those rounds?

I've pushed a few to the point of extractor marks, but only blown a primer once.

He said it was a Ruger No. 1

I think you meant to say ejector and No 1's don't have 'em

I've blown one. Ruger No. 1 also - 180 gr bullet in a .30 Gibbs ('06 super improved) using RL22. Chrono read 3,150 fps. Primer fell right out.

I had shot this load before. Dumped that bottle of '22 lickity split and have never used the stuff again.

Well, that would explain why he didn't check for extractoor marks.......

Ackley experimented with blown primers.

His test equipment was a copper crusher setup, so his results were actually in what we now call CUP. But, as was common at that time, he reported them as PSI.

Doing the necessary units conversion, he found that primers start falling out around 80 KPSI, which is consistent with MD's result.

My testing has shown about the same. A chamber cut to spec, and an action machined for a correct casehead to boltface interface, pressure in the mid 70's didnt show anything.
Charlie

As Mr. Sisk and others have noted if all things are right then...

I believe that I had a blown primer on a rifle that I purchased on-line and the firing pin looked like a flat punch with sharp edges. I am pretty sure that the pressure wasn't the cause for the blown primer. I replaced the firing pin and had no further problems. I suppose that there are other things besides pressure alone that will cause blown primers.

I'm not an expert as some others are that have answered this thread, so take my comments as my two bits.

It's not the same for all primers. This isn't a rifle-specific area of the forum, so I'll add that I don't think standard pistol primers will take 80kpsi or even 65kpsi. Magnum pistol primers or non-magnum ones with a thicker cup (Remington 5 1/2) can take over 50kpsi in cartridges like the 5.7x28, but higher pressure cartridges like the 454 Casull (65kpsi) are specified with rifle primers. I don't know what the ultimate limits of pistol primers are, but I (unintentionally due to a scale use error) fired some standard SPP at a pressure that Quickload calculated to be somewhere between 51 and 53 kpsi. The primers were bulging and cratered with a sharp metal ridge raised around the pin dent. They were not flat, but ballooned out. They probably would have been fine if they were thicker magnum pistol primers, but as it was, the cup must have been close to failure. By close, I think another 15kpsi would have resulted in rupture.

There are different things that people call "blown" primers....

1. The primer pocket expands to the point that the primer falls out. Had a shooter at our range that experienced this with factory loaded 270 ammunition.

2. The primer flows out of the pocket excessively.

3. Cartridge pressure causes the firing pin dent in the primer to invert, forming a bubble instead of a dent. With just a little more pressure, the bubble cracks around its base and falls off, leaving a "pierced primer", which isn't really pierced at all. The bubble then often falls into the mechanism. This can happen in an AR15 that is firing 5.56 level loads that use standard small rifle primers.

4. An oversize firing pin hole allows the formation of a crater around the firing pin dent in the primer.

5. Propellant gas leagage around the primer.

Ackley's experiment was with #1.

So how much pressure is needed to blow up a modern bolt action rifle? If primers go POP at 80 kpsi how much more pressure is needed to destroy the action?

I've had primers fall right out of numerous different cartridges in several different rifles. Some (mostly 300WSM's) were factory loads, some were loads on the edge, and some were baffling. When I was shooting a 6x47 for short range BR (for those who don't know, the 6x47 is the 222Mag necked up), the best accuracy was a load which loosened primer pockets pretty quickly. On a hot day, I would occasionally drop a primer. In my 256 Newton, a load which should have been fine, suddenly blew a primer. I later found that I was having trouble with powder hanging up in the measure and I suspect I had one light charge followed by a heavy one. My carelessness. This was in a Model 54 Winchester and I was happy not to have gotten any gas in my face. As it was, there was a puff of smoke and that was it.
I have loaded rimmed cartridges (303 British and 30/40 Krag) damned hot and have not blown a primer. I have some thoughts on this but they're just my thoughts and may have no relationship with reality. GD

I thought the size of the primer hole in the case also affects the pressure/force seen by the primer. I realise the difference between pressure and force BTW.

I've only seen pierced primers (faulty primer), leaking primers and experienced loose primers (was a batch of undersized Winchester ptrimers). The leaking primers were in Winchester factory 8x57 ammo (oldish unmarked blue box ammo) that eroded the bolt face - someone elses rifle.

I haven't persnally experienced blown primers and assumed the term was more often used for overpressure loads and case head expansion. The pierced primer was bad but not catastrophic.

I've had two primers to pierce in twenty five years of reloading. They were Remington 9 1/2 primers. My friend was using the same lot of primers and he had a primer to pierce.

I don't know exactly where that demon lives, and I don't want to experimentally find out. It's a lot, though.

Just as a reference point, the US Mint puts 160,000 PSI pressure on the steel dies used to form high grade coins. After many uses, they do tend to crack.

So what's the point of pushing a load to that level ? Wrecking your gun ? Getting hurt ?

Depends.....

I realize that's not a popular answer but what you're describing could depend on a number of factors, such as, the peak maximum pressure, the maximum average pressure, the condition of the primer (thickness, quality of material), the firing pin (I've seen folks sharpen their's for better ignition 😵). The quality and/or design of the action or chamber, any strange restrictions (i. e. really tight chamber, no free bore, etc...)

So it depends.

Going back to your original post you used 65,000 psi as an example but if you look at the SAAMI spec sheet for centerfire rifles you'll see that SAAMI rifle chamber pressures vary with cartridges and so the proof loads also vary.
https://saami.org/technical-information/ansi-saami-standards/

On page 352 you'll see how SAAMI goes about "proofing" cartridges for safe use. The basic rule of thumb is between 1.3 to 1.4 times above your safe load. For an example, if your rifle load lists 50K psi as a max safe load then it was probably Proofed at 65-70K psi. Don't try and find out when it will explode because there's a lot of variables that will be different for every different rifle/cartridge combo you run into. That's why having standards is good.
Remember, safe loads are designed to last the life of the barrel, going higher will certainly shorten the life of your barrel and possibly put you in danger. Stay safe 👍

Cowbell,

Actually, SAAMI maximum average pressures (MAPS) never exceed 65,000 PSI in any cartridge, though quite a few rifle rounds have lower MAPs.

65,000 was established as the maximum for ANY round not to increase barrel life, but to ensure that outside factors (primarily warmer temperatures" wouldn't cause pressures to rise to dangerous levels. Even with the most temperature resistant powders made today, higher temperatures cause pressures to rise, whether the heat is a result of amient temperature or a barrel heated-up by firing. The pressure-rise is less than with "standard" powders, but still rises. If the round gets hot enough, pressures can indeed rise enough to cause several problems--but barrel life is not a consideration.

Then there are tougher primers than most. I bought a couple thousand CCI No. 34 Primers for 7.62mm Ammunition (Yeh, I know a fancy military 308)

They are supposed to have a tougher cup. They were tougher, I understand because of unintentional firing of ammo when an AR slams the bolt shut. But I'd think they would also take a higher pressure before they would vent.