Primer behavior........

[BobWright]

Under the thread "Most authentic SAA clone?" the learned, and experienced, Mr. Driftwood Johnson made this statement:

All primers back out of the primer pocket as a round is fired. Then as the round slams back against the frame in recoil the primer is reseated.

This is an interesting observation, and peculiar to revolvers. In most other firearms, the cartridge is held firmly against the breech face, but in the revolver, clearance must be provided to insure smooth cylinder rotation.

Anyone who has ever loaded wax, rubber, or plastic bullets propelled only by the primer will have likely experienced this. When I was a very young kid I loaded wax bullets into .45 Colt cases for use in my bedroom, where I had a makeshift range. I fired one shot, and could not rotate the cylinder. Examination of my gun and fired case showed the primer had backed out and bound up the gun. The pressure inside the primer pocket and cup had driven the primer out and flowed around the firing pin. Without a powder charge, no pressure drove the case back to reseat it. The way around this is. of course, to drill out the flash hole in the case. This will work for primer actuated ammuntion, but renders the case useless for loading with a powder charge.

Not only does the primer charge build up pressure, but some of the pressre from the burning powder charge leaks through the flash hole. We can read, to some extent, the pressure generated by our hand loads. In most cartridges, the firing pin will leave a deep indentation, well defined. As pressure increases, the pressure will push the firing pin back slightly, and the primer indentation will be less deep and defined. Very high pressure will flatten the primer, throwing up a rim where the primer flows in the countersink around the primer pocket. The primer will also have an imprint of the breech face, and a fainter firong pin indentation.

In view of the foregoing, the fired primer is a pretty good indication of escalating pressures. Its telling the loader to proceed with caution. Be aware its not an infallible sign, but a good one to observe, along with good common sense and a good reloading manual.

Bob Wright

 

[Haxby]

The same thing happens with light loads in other types of firearms, too. I've had primers backed out in bolt action rifles.

 

[BobWright]

I'm not much on long guns, but if a primer backs out in a bolt gun, I'd suspect excessive headspace?

Bob Wright

 

[Haxby]

Or maybe exactly the same thing that Driftwood described.

 

[MrBorland]

In most cartridges, the firing pin will leave a deep indentation, well defined

I believe Bob's point here is that the primer on a fired case that was within normal pressure limits will have a deep indentation in it.

Just to clarify a minor point, though - the initial firing pin strike isn't what creates the deep indentation we associate with a fired case. The initial FP strike actually produces a fairly shallow dimple. The indentation on a fired case is deep because of the reset effect Bob described; that being the case/primer slamming into the firing pin on it's reset.

I only point this out because it's tempting to see a small dimple on a round that didn't go off and assume the small dimple points to a "light strike", when it may very possibly just reflect that the round simply didn't go off.

 

[Zaydok Allen]

I only point this out because it's tempting to see a small dimple on a round that didn't go off and assume the small dimple points to a "light strike", when it may very possibly just reflect that the round simply didn't go off.

That is an excellent point to keep in mind. It may be an entirely different reason that the cartridge didn't fire.

 

[Driftwood Johnson]

Howdy

Yes, the first time I noticed primers backing out was also shooting wax bullets in my 45 Colt Blackhawk about 40 years ago. Took me a while to figure out what was going on and why the revolver kept locking up. Even today, sometimes I will pop a cap on just a primer to make sure a thinned hammer spring is strong enough. I have to remember that I may have to persuade the cylinder to turn once I have popped the cap.

In the CAS world, sometimes shooters load their ammo ridiculously light, and the rounds don't generate enough recoil to reseat the primer, tying the gun up.

When I am Trap shooting with my old Winchester Model 12 sometimes I have a round that does not fire. Commercial ammo, not reloads. When I take the round out to look at the primer, the dent is usually quite shallow, indicating to me anyway that it was a light strike. They always go off the second time.

But it is certainly true that examining primers can be a good indicator of too much pressure.

 

[murf]

factory and full-sized rifle rounds are not held firmly against the breech and have a bit of play inside the chamber (neck sized rounds probably not). so, rifle cases do this forward and back routine when fired, too.

this is the only disagreement i have with the op. except that powder gases always leak back through the flash hole (sometimes enough to rupture the primer and blast the gas back through the action, or cock the hammer on a single action).

i have a 9mm round that didn't go off sitting here next to a fired round out of the same gun. the firing pin indent on the dud round is larger and deeper than the fired round. just like bobwright described.

murf

 

[Walkalong]

It happens in all chambers. There has to be some clearance and that is what allows the primer to back out just a bit, but then the case stretches back to the breech re-seating the primer.

 

[jmorris]

This is an interesting observation, and peculiar to revolvers. In most other firearms, the cartridge is held firmly against the breech face, but in the revolver, clearance must be provided to insure smooth cylinder rotation.

Anyone who has ever loaded wax, rubber, or plastic bullets propelled only by the primer will have likely experienced this. When I was a very young kid I loaded wax bullets into .45 Colt cases for use in my bedroom, where I had a makeshift range. I fired one shot, and could not rotate the cylinder. Examination of my gun and fired case showed the primer had backed out and bound up the gun. The pressure inside the primer pocket and cup had driven the primer out and flowed around the firing pin. Without a powder charge, no pressure drove the case back to reseat it.

You are assuming to not have an equal and opposite reaction. With a projectile that has no weight and little force pushing it away can extract the primer. You can fire an empty case, with a primer only and see the same result. Drill out the flash hole and the primer will not self eject (don't use these for "normal" loads). The primer force is pushing it out of the pocket due to the small flash hole and resulting pressure.

If you have a load that will push a lead bullet out of the barrel, I haven't seen primers pushed out, unless they would have fell out of the pocket anyway.

You can see the same thing with Speer plastic bullets/cases where you can push the primers out by hand but they don't set back.

 

[Vern Humphrey]

I'm not much on long guns, but if a primer backs out in a bolt gun, I'd suspect excessive headspace?

More likely excessive pressure.

With excess headspace, the head will usually move backward under pressure and reseat the primer, but at the same time stretch the case wall just forward of the web. You will often see a bright ring there, and be able to feel a groove or ring inside the case by feeling with a bent paperclip.

 

[Jim K]

Driftwood's original statement is correct. But that happens with all (repeat, all) guns. Every gun has to have some headspace or some cartridges could not be chambered. And that space will allow a primer to back out to some extent.

The confusion comes in because there are two sources of internal pressure in a primer. The first is the pressure generated by the explosion (that is the correct term) of the priming compound as it is set off by the firing pin. The other is the pressure of the gas generated by the burning powder as it pushes back through the flash hole.

The first pressure will push the primer back as far as it can. Then the second pressure pushes the case back over the primer so when we look at a fired case we don't normally see any sign that the primer moved back. But if the second pressure (from the burning powder) is very low, or non-existent (there is no powder), then the primer will protrude because the case has never been pushed back over it.

Jim

 

[BobWright]

For the record, I once did this experiment, and before doing so, put on heavy gloves and safety glasses.

I placed a primed empty .45 Colt case mouth down in a vise, letting the rim rest against top of the vise jaws. Using a Lee decapping rod/pin, the kind that came with the old hammer drive set, I struck the primer with the pin in much the same way the firing pin would, being careful not to puncture the primer. The resulting explosion drove the primer completely out of the primer pocket impaling itself on the decapping pin. the primer was pushed all the way to the end of the pin, up against the end of the rod.

That's a pretty hefty explosion in that little cup.

Bob Wright

 

[Vern Humphrey]

John Garand's original design was for a primer-actuated action. The primer was blocked by a weight, and when the primer backed out, the weight was driven back, unlocking the weapon.

Brian Pearce wrote a column in either Rifle or Handloader Colt SAA going full automatic. It was a pre-War .357 Mag, which had the old large diameter firing pin and the primer backed out, metal flowed into the firing pin hole, pierced, and the gas pressure blew the hammer to full cock. Since his finger was still on the trigger, the hammer fell, and fired the gun again.

 

[murf]

back when i was turning a wrench, one of the mechanics took a primed case, clamped the rim in a vise, took a hammer and punch and "deprimed" the case. the primer exploded, came out around the punch, entered the guys skin just right of the bellybutton and traveled about a half inch under the skin. you could see the bump of the primer just under the skin. he dug the primer out and went about his business, but i got a good story out of it.

i was standing next to him when he did this, so this is first-hand knowledge. primers have my respect.

murf

 

[Zaydok Allen]

Hey gents, I hope this isn't too far off topic, but it seems like a simple answer could be given, and isn't worth starting a new thread. And we are talking about primers, so.....

I am in the process of setting up a reloading bench in my attic. It's a pitched roof and short. I can't stand completely upright in the middle as it is only about 5'6", and slopes down to about 3" on the sides, but there is plenty of room for work tables to sit at. It is carpeted, and has a tongue and groove pine ceiling and walls. So it makes for a very pleasant work space, and is just spacious enough to sit and work on the computer, clean a gun, or load bullets.

Anyway, I've been considering a Dillon progressive press. However, I've read one too many stories of primers detonating in the primer feed tubes in a rather explosive fashion. Given that there is a chance of that happening, and the pine boards on the ceiling, , and the fact that the top of the feed tube will only be about 3 inches from the celling, do I need to screw in a metal plate into the pine to protect it? It's my understanding that a detonating tube of primers can easily punch a hole in a wooden board. Or is that over statements on the part of story tellers.

I know some of you all reload, so I thought I'd ask since we are talking primers. If a mod thinks I should start a separate thread, please let me know, and I'll do so.

 

[eldon519]

Primers can be useful in some situations for reading pressure signs, but they are not very reliable. The effect varies brand to brand, for instance CCI SP primers will completely flatten if you so much as sneeze in their presence. Also it depends on the spec pressure of the caliber as well. For instance, you might explosively disassemble something like a .44 Charter Bulldog before you ever got flattened or dimpled primers while in contrast, flattened primers in a .454 Casull are just normal.

 

[Jim K]

Hi, Kodiak,

I have loaded many thousand rounds with a Dillon and have used other presses with priming tubes, and have never had any primer go off. I have heard some stories, but have never seen that happen or personally know of anyone who has had it happen to them. The only way it could reasonably happen is if the press operator applied a very sudden and very forceful stroke in the priming process, at which point the priming arm is not under the tube, but under the case. Even then, the blow would have to be very quick, so much so that I don't think a human being could move fast enough.

I would not worry about the priming tube "going off", but if you think it might happen, put a piece of steel or maybe a piece of plywood on the inside of the roof. Don't try to seal off the tube; if the primers did ignite, that would confine the explosion and convert the tube into a bomb, spraying fragments all over.


Back on the primer appearance. The flattening of a primer is not caused at the first stage, when only the pressure from the primer itself is involved. It comes after the primer has fired and backed up, then the pressure from the burning powder pushes back through the flash hole, flattening the primer and spreading it out into the gap between the primer and the wall of the primer pocket. Since the primer pocket has a rounded rim, the primer flowing into that area will appear larger and flattened. Because that is due to the pressure generated by the powder charge, it is relative to the pressure in the case and (all other things being equal) primer flattening is a reasonable (though not infallible) indicator of excess pressure.

Jim

 

[Zaydok Allen]

Thanks Jim.