Primer question

[chris in va]

I don't think I've ever seen this addressed before.

The primer fires, and a jet of flame shoots into the powder. Pressure builds and the brass expands, sealing the chamber.

Now, does powder gas flow back through the flash hole and expand the primer, sealing against the case primer 'cup' (for lack of proper term)?

 

[Jimro]

Your understanding is essentially correct.

The laws of gas pressure ensure that the pressure inside the primer cavity are the same as in the main portion of brass, barring differences caused by flow restrictions.

The pressure will push the primer out of the primer pocket and back up against the bolt face, then the pressure will stretch the cartridge brass to fill all available space between the shoulder and the bolt face, pushing the primer flush with the case head. If there is a low pressure load, where the gas pressure can push out the primer but not have enough pressure to stretch the case, you will see pushed out primers.

For a high pressure instance, the brass will stretch making the primer flush with the case head, but then the thinner metal of the primer will flow outward and flatten against the primer pocket. This "flattened" primer is a sure sign of high pressure and why high pressure magnum rounds use primers with thicker metal.

Hope this helps.

Jimro

 

[Bart B.]

Here's the sequence of events:

1. Firing pin smacks primer driving a .308 Win case hard into the chamber shoulder setting it back a few thousandths. The case head is now a few thousandths further away from the bolt face.

2. Primer detonates, burns powder, case body expands first in its front half against the chamber wall holding it there after pulling case neck back into the shoulder area.

3. Primer cup gets that pressure inside and it pushes back against the bolt face; sometimes several thousandths out past the case head; often flattening it quite a bit.

4. Chamber pressure increases to maximum pushing the back half of the case rearwards until the case head stops against the bolt face. This pushes the primer back in flush with the case head.

5. Chamber pressure drops to zero after the bullet leaves and the case outside shrinks back a couple thousandths from the chamber walls.

The fired case ends up a few thousandths shorter in length than it was before firing.

 

[Jimro]

Bart B.,

I seem to remember you writing once that you had a Garand and a bolt rifle shooting the same load, with barrels from the same maker and chambers cut from the same reamer, but the Garand empties wouldn't fit back into either chamber but the bolt empties would easily chamber....

Jimro

 

[Bart B.]

Jimro, right you are.

I should have mentioned in my post on what happens is it applies to bolt guns.

Garands, as with a lot of semiauto's, have empties larger sized for the same chamber dimensions as bolt guns. I shot with a guy in California who's Win. 70 had the same .308 Win. chamber as his Garand; both were made with the same reamer. Empties from his Garand wouldn't fit in either chamber. Bolt gun empties would. Same load with new cases in each situation. Garand's cases had about half an inch past the chamber back edge in both rifles

I think that happens because the Garand's cases are ejected much sooner after firing than bolt gun ones. They are hotter and for some reason don't shrink as much from dimensions at max pressure.

 

[Unclenick]

All I have to add to the above is that the flow constriction in the flash hole is enough and the primer brissance high enough that the primer actually backs out under its own pressure before the powder burn pressure builds. This is why flash holes have to be drilled bigger for primer-fired wax and plastic bullets. If you don't do that the primers will set back enough under their own steam to jam a revolver cylinder. It's also the reason larger flash holes were in the NT cases made for large pistol lead-free primers. The DDNT sensitizer in them has still higher brissance than lead styphnate, so they make their gas even more suddenly and could actually pierce or leak without the larger hole to let them vent gas faster.

From the above, in a rifle, I expect the primer pocket sees two pressure peaks, the first being from the primer itself, which starts bleeding down, but then pressure builds again as backflow from the powder gas starts to occur. That backflow develops even more pressure than the primer did initially, because it is what causes the primer cup flow in an overpressure round that mushrooms or is cratered.

The gas guns seem to start opening before the brass has finished unsticking from the chamber walls. It's common to see bent rims pulled back by the extractor.

 

[sawdustdad]

Thanks for the question and explanations. All very logical in terms of sequence of events. Learned something today. What a great place this is.

 

[44 AMP]

I think that happens because the Garand's cases are ejected much sooner after firing than bolt gun ones. They are hotter and for some reason don't shrink as much from dimensions at max pressure.

I'm no expert, so this is just a SWAG, but it might be that there is still enough pressure in the case as it is being extracted by the Garand to stretch it as it is being pulled out of the chamber, and it then cools and springs back to the "larger" size.

No matter how fast you work a bolt gun the case has stretched and sprung back as much as its going to by the time you get the bolt unlocked.

flash holes have to be drilled bigger for primer-fired wax and plastic bullets. If you don't do that the primers will set back enough under their own steam to jam a revolver cylinder.

Absolutely! I have done this, sort of. Made some primer only "blanks" for my .45 Colt. Paper wad. Standard cases. Primers did back out!

 

[oldandslow]

Bart,

Your explanation makes sense, but one question about your post #3. You mention that "the fired case ends up a few thousandths shorter in length than it was before firing."

So I'm confused. All my bottle-necked rifle cartridges grow in length after firing and have to be trimmed back to the optimal length (30/30, 30-06, .270 Win, .223/556). How is it that your's shrink in length?

best wishes- oldandslow

 

[Bart B.]

44Amp, Garand bolts are fully unlocked when their bullet's are about 7 feet out past the muzzle. I doubt there's enough pressure in the case to press it against the chamber wall. That's not easy to measure. If there was, I think there would be lengthwise scratches on the case as it's pulled out of the chamber.

oldandslow, I measured case lengths before and after firing about 4 dozen shots with a .308 bolt gun. Starting out with a new Federal case 2.000" long, it was loaded then fired with a max load under 165-gr. bullets. After firing, the case was about 1.997" long; shortened about .003". Full length sizing it made it grow to about 2.001". Loading it then firing it again had the same shortening and growing cycle each time; gained close to .001" every time. When it got up to about 2.010", I trimmed it back to 2.000". My .30-.338 Win Mag cases had about .004" shorter length when fired, then grew a bit over .005" when full length sized.

I think most people measure case length after it's been sized. That causes the brass to grow and the only way it grows while in the die is longer. When fired, the case starts out smaller then gets larger when fired; that has to take metal out of the case neck to move the case body to a larger diameter.

Jimro, I don't think there's any difference in primer cup thickness between standard and magnum-labeled primers:

http://www.accurateshooter.com/technical-articles/primers-and-pressure-analysis/

I think Tulammo's (formerly PMC then Wolf) magnum primers are thicker than their standard ones, so I've read on one of their web site pages. But the primer pellet's the same.

 

[Unclenick]

Ditto to Bart's explanation of stretching. Below is an exaggerated illustration. If your chamber is wider than the case (normal), expansion can draw brass in from the neck. You can tell that's the cause because it doesn't happen with neck sizing only.

I think I Bart is right that residual pressure at the back end of the chamber is not playing a significant role in case stretching by extraction in the gas guns. I own a Pressure Trace and have seen to my own satisfaction that pressure drops too fast for that to be the case. This would be especially true in a Garand where the gas system doesn't start going to work until the bullet is already about at the muzzle. Instead, I think this is just a delay in the unlocking of the brass due to the fact its tendency to spring back to size doesn't have a lot of force. As a result, it takes time to accelerate the brass mass inward and to peel itself off the chamber wall, where the same molecular adhesion that drives the difference between static and kinetic coefficients of friction will have occurred. That's when the extractor begins to succeed in moving the case rearward. Once it has released from the chamber wall, the rest of the case's return to size would be pretty quick, and, specifically, quicker than the case has moved much to the rear. Hence no longitudinal scratches.

 

[Bart B.]

For good info on M1 and M14 cycle timing, read post #4 in:

http://thefiringline.com/forums/showthread.php?t=552893

 

[44 AMP]

OK, I accept there isn't enough pressure to expand the case during extraction. SO why doesn't the fired case fit back into the chamber it was fired from? (this is what I understand is happening)

Are you saying the brass "sticks" to the chamber walls (at the front?) and stretches as it is pulled on by the extractor, the "lets go" (springback) all at once and the extractor pulls it out clean (no scratches)?

Would the M1A (M14) do this as well? I admit I've never tried putting one of my fired cases back into the chamber of my M1A (prior to resizing), it never occurred to me that they might not fit...

I've never seen a manually operated action where fired cases won't fit back into the chamber they were fired from. And I've never noticed it happen to any of my semi autos. Something I'll have to check on, I guess...

 

[Bart B.]

44AMP, I don't know all of the exact case to chamber differences in the whole cycle at different timing points.

I've tried to rechamber once fired match and service 7.62 NATO cases in both M1 and M14 rifles; they would not go back in. Same lots of ammo fired in bolt guns with slightly smaller chamber diameters would let the once fired case easily slip fully into the chamber they were fired in.

To me, this is another "Houdini Syndrome." Something magical happens that's beyond my ability to figure it out. Something causes fired cases to end up larger in diameter from these rifles compared to bolt guns.

I might bet a little pocket change that if the gas plug on a Garand was removed and a round fired without the op rod cycling the bolt pulling the case out, that case would go back into the chamber easily.

 

[mdmtj]

I think that happens because the Garand's cases are ejected much sooner after firing than bolt gun ones. They are hotter and for some reason don't shrink as much from dimensions at max pressure.

The gas guns seem to start opening before the brass has finished unsticking from the chamber walls. It's common to see bent rims pulled back by the extractor.

I might bet a little pocket change that if the gas plug on a Garand was removed and a round fired without the op rod cycling the bolt pulling the case out, that case would go back into the chamber easily.

Been there, done that, case went into chamber with no problem

Some where in my travels I had it explained to me. An auto action begins extracting the case far sooner than the case is extracted from a bolt action. This results in the case still expanding as it is extracted from the chamber and ends up expanding more than the case from the bolt action. This case does not have the constraints on it as it expands that the case from the bolt action has. As stated, the case from the auto action expands more and when it cools it doesn't contract as much resulting in a case that is bigger than the chamber. The expansion of the case in the bolt action is limited by the chamber and when it cools and contracts it ends up smaller than the chamber.