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Discussion Starter #1
Thinking about the firing sequence in revolvers is complex. I cannot understand the part about when the primer backs out of the case. As the primer ignites the powder, pressure inside the cartridge increases in all directions. Does the primer back out of the case because the bullet has not moved forward yet? Once there is forward motion of the bullet, the whole case will move rearward, striking the recoil shield.

Has anyone taken high speed video of the primers being blown out of the case?

Thank You!
Eor
 

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Eor, You can simulate the condition by loading a spent case with just a primer ... no powder or bullet. Fire the case in your revolver then remove it and inspect the primer. It should be a tad high. Because the flash hole is so small, pressure generated by the primer flash will back the primer out until it contacts the recoil shield. You can measure the case length at the center to include the high primer then measure off to the side a little to omit the primer. The difference in measurements will be your exact headspace .... about .010"

In a normal cartridge .... there is a very slight time delay after the primer flashes but before the powder ignites and chamber pressure builds. At that time, the case head will be thrust against the recoil shield with thousands of psi. This will reseat the primer where it is flush with the end of the case.

Look at post #2 in this thread for the full sequence: http://rugerforum.net/gunsmithing/68961-b-c-cylinder-gap-measuring.html
 

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You also factor in The make of the case and the brand of primer.
 

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winggunner46, It happens with all primers and cases no matter what brand.
 

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Discussion Starter #5
Thanks. This process can get difficult to visualize sometimes. Does this same thing happen regardless of action type? I never heard anyone talking about primers backing out in semi-autos, for example.

Eor
 

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Eor,
Does this same thing happen regardless of action type?
Yes, it's all about headspace and the fact that small flash holes restrict pressure created when the primer detonates. It happens in all guns except rimfires (because there is no primer). First, let's define headspace ... which is the distance between the case head and the breach face or recoil shield. Nearly all revolvers have some "endshake", which is fore and aft cylinder movement. Endshake adds more headspace as the cylinder is pushed forward when the primer detonates. So ... the primer will back out to headspace plus endshake. In a normal revolver, this will drive the primer back no more than .015"

You can do the above test with any gun and find the primers will back out until they contact the breach face (or recoil shield) which will stop the primer's rearward movement.

Typical centerfire rifles have pretty tight headspace so primers won't back out more than .005" ... hardly detectable. Pistols tend to have more "effective" headspace than revolvers so a backed out primer is actually easier to see and measure than those fired in a revolver.

Why?

Revolver cartridges have a rim that prevents the cartridge from seating deeper in the chamber. As such, headspace will typically be between .008" and .012" with .010" being optimum and a few thousandths of endshake for a worst case total of about .015". So ... primers will back out somewhere between .010" and .015" ... depending on actual specs for a specific revolver.

Semi-auto pistols are also designed to have about .010" headspace, however the chambers are designed much different, making headspace dependent on case length. If you look in any semi-auto pistol chamber, you will see a "case mouth stop". This is a ring inside the chamber that contacts the case mouth and prevents the case from chambering deeper. This stop is cut to SAAMI specs for the maximum case length for the specific cartridge. As an example .... a 45 ACP case has a max SAAMI length of .898" so the typical chamber will be cut to .900" deep plus there will be an additional .008" of headspace making the total headspace with a full length case about .010". Herein lies the problem. Very few 45 ACP cases actually measure full length ... in fact .888" is quite normal (.010" under SAAMI spec). When you account for normal headspace then add .010" for a shorter case, it is quite normal to see a .020" combined headspace, which would be how far the primer backs out.

To expound a little more on primer back out .... a typical primer develops about 1000 psi when detonated. The flash holes in large and small primer pockets are pretty standard at 5/64". Large primer diameters measure just over 3/16" and small primers measure just over 5/32". So what happens is ... a half ton of pressure is trying to be forced through a small hole in a light weight cartridge, which will force the primer to back out until something stops it (breach face or recoil shield). Primers are actually designed for "back out" by making the length of the primer long enough where it is never pushed out so far that it will be blown totally out of the pocket.

Another test will prove this by drilling out the flash hole in a small primer pocket to 9/64" or 3/16" for a large primer pocket. When the flash hole is larger, the pressure generated by the primer flash will vent through the flash hole and NOT make the primer back out. BTW, I use this technique with "test cases" when checking a revolver for proper hammer energy. If you pop off primed cases without enlarging the flash holes, just one or two "pushed back" primers will lock up the cylinder and prevent it from rotating. Further, popping primed cases that have not been drilled out is the most accurate way to measure headspace in any centerfire gun.

Hope this helps without being too confusing.
 

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Another aspect of primers backing out is when pressures are way too high.
Then the primer is actually mashed and flattened, sometimes "overflowing" beyond the primer pocket. If you see this happen, it's definitely an "uh oh!" moment even if the gun doesn't show damage. For a handloader this is definiiely a time to review loading practices.
 

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Another aspect of primers backing out is when pressures are way too high.
Then the primer is actually mashed and flattened, sometimes "overflowing" beyond the primer pocket. If you see this happen, it's definitely an "uh oh!" moment even if the gun doesn't show damage. For a handloader this is definiiely a time to review loading practices.
You'll get a lot of false positives trying to read federal primers that way though.
 

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Discussion Starter #9
Thank you Iowegan. That is fascinating information. I have never heard of the method of testing you describe, that is, firing a primed case to check dynamic headspace. I'll experiment with it. I believe that the "teardrop" effect on semi-auto primers may have a lot to do with the primer backing out. I always figured the teardrop was made because the firing pin could not retract fast enough. This testing method also puts true revolver headspace in perspective.

Eor
 

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lowegan,

Being new to the world of guns and shooting I thoroughly enjoyed your "lesson". One of the primary reasons I come to the forum is to learn and this was a good mini-lecture. I look forward to hearing other discussions from you in the future. Thanks
 

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Eor, Yes, some pistols do have firing pins that don't retract fast enough. This is very common in Ruger P-345s, all Glocks, and a some other delayed blow back actions where a drag line or "tear drop" mark on a spent primer is very obvious. It doesn't seem to hurt anything and it is related to primer back-out.

I made a mistake above where I said "It happens in all guns except rimfires" ... there always seems to be an exception and in this case it's military brass where primers are staked in place. The staked brass edge of the primer pocket will prevent primers from backing out. This is done to eliminate primers from totally blowing out and ending up in the action, which could cause the weapon to jam. Once these fired military cases have the primer pockets swaged, they behave just like any other commercial ammo allowing primers to back out.

Just some info on measuring headspace ... with revolvers, this is normally done with a feeler gauge located between the case head and the recoil shield directly in front of the firing pin hole. By holding the cylinder fully to the rear, the ratchet column will contact the recoil shield and establish "true headspace". However if there is any cylinder endshake (which there almost always is), you have to hold the cylinder forward and measure again to get "total headspace". If total headspace is excessive, the firing pin will "push" the cartridge forward, which will cushion the blow and may cause a misfire. So ... true headspace must be enough where case heads won't drag on the recoil shield and total headspace must not be too much or you will get misfires and in extreme cases, the cylinder may unlatch when the revolver is fired ... a very bad condition. Popping a primed case and doing a "before and after" case length measurement over the primer is very quick, cheap, and conclusive for "total headspace".

Measuring headspace in a pistol requires just one measurement because there is no "endshake". What you do is field strip the pistol then place the barrel (and bushing if so equipped) back in the slide with a spent case chambered (no primer). With the slide belly up and the barrel held locked in the slide, use a feeler gauge to measure the distance between the case head and breach face. This takes special feeler gauge blades that are narrow enough to slide in place. Once the headspace gap has been measured and recorded, you have to measure the length of the case and do a little math. Using the above .888" 45 ACP as an example ... subtract that from a full length case (.898") which is .010" and subtract that .010" from the thickness of the feeler gauge. The end result will be headspace with a full length case. Popping a primed case is even easier. Just measure the case length over the primer after it has been popped and subtract .898". No need to strip the gun down or buy special feeler gauges.

Rifles are very difficult to measure headspace because you can't access the case head. You can buy special headspace gauges (they look like a cartridge) but they are expensive and are unique to a specific cartridge. Here's where popping a primer works exceptionally well ... it's very quick, cheap, and accurate. Merely do a before and after case length measurement on a primed and sized case and subtract the difference.
 

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Rifles are very difficult to measure headspace because you can't access the case head. You can buy special headspace gauges (they look like a cartridge) but they are expensive and are unique to a specific cartridge. Here's where popping a primer works exceptionally well ... it's very quick, cheap, and accurate. Merely do a before and after case length measurement on a primed and sized case and subtract the difference.
I'm going to have to try that.
Have several old mismatched mausers that I sort of checked the headspace using tape on the base of unfired cartridges....
 
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