Loose Primer Pockets

[Robinhood]

Loose primer pockets. Often people say that over pressure loosens primer pockets. While that may be true, more often the culprit is an oversized chamber combined with high pressure loads. If a reamer is out of spec or the tailstock center-line is off just a few thousands of an ", it can cause the base of the chamber to be cut OS. Over polishing could be the culprit also.

In this drawing you can't match up exact numbers but you will see the case major diameter at the base is .4703". This is about .070" in from the breach face when chambered. At .200" into the chamber the .4714. A difference of about .0011" on diameter. calculating the distance and taper of the 308 case that .070" equals to about .0008 on diameter. Therefore your SAAMI spec'ed chamber should give you less than .002" between the chamber and the base of the case. In a perfect world...........Just something to think about.

[bigedp51]

On the vast majority of rifle cartridges the very base of the case is not supported with the exception of rimmed cases. Meaning a over sized chamber would cause a bulge where the case does contact the chamber. "BUT" high pressure will have a effect on primer pocket life.




One of the biggest problems of short primer pocket life is the thickness of the flash hole web and the hardness of the brass. Below you can see why Federal get a bad rap for oversized primer pockets after a few reloadings

.

Below Lake City and contract ammunition for the military uses harder brass and thicker flash hole webs.





At the link below measuring the expansion just above the extractor groove doesn't tell you the chamber pressure. It just tells you how soft the brass is and the strength of the flash hole web. Meaning the same load in a Federal case would show more base expansion than a Lake City case.

Simple Trick for Monitoring Pressure of Your Rifle Reloads

http://www.hodgdonreloading.com/relo...-rifle-reloads

[Robinhood]

He ed! Good to hear from you. Neat graphics that skew the correct information. If you look at your first graphic you will see your argument is counter intuitive. The primer pocket depth is within .010" of the cartridge protrusion. You then have approximately .065 above the primer for the flashhole. So the graphic is not only misleading, it is incorrect.

Draw the breach face in a straight line at .010 above the bottom of the primer pocket rather than showing .300" protrusion and you will get the real story.The bulge cannot take place there because it is captured by the chamber....unless the chamber is ...oversized. I had read all of that information before from the same place you got it. It did not stand the test then and it does not now. Sometime pictures explain things and sometimes they tell lies.

Your graphic tells the tale of incorrect protrusion. Nothing else.

[Robinhood]

For your comparrison

[bigedp51]

Sometime pictures explain things and sometimes they tell lies.

Your graphic tells the tale of incorrect protrusion. Nothing else.

Darn, I looked on the internet for hours trying to prove you wrong and couldn't find anything.

But I still decided to not invite you to my birthday party this year anyway.

Protrusion aside I'm only half wrong, below is a photo from AR15.com and the reloading moderator. A two inch rod was used to measure the flash hole web thickness. The base of the rod was drilled and cup shaped so any protruding burs would not give a false reading. Any Federal or other cases with a thin flash hole web were tossed in the scrap brass bucket as not worth reloading. I had factory loaded 09 and 10 dated Federal cases with loose primer pockets after the first firing. The two inch rod saved you from sizing and prepping the cases only to find loose primer pockets when seating the primers.

I made up my own two inch rod to check my Federal cases but then I said why bother inspecting all theses cases and just started using nothing but Lake City brass. That being said it is how hard the brass is in the base and the thickness of the flash hole web that governs primer pocket life.

longevity

Below both these .303 British cases were fired in my 1943 No.4 Enfield rifle with a large diameter chamber. The Prvi case is larger in diameter and .010 thicker in the base than the HXP case. And my point here is the case construction has a great deal to do with primer pocket and case longevity even with large diameter military chambers.



During the 1968 Congressional hearings on the M16 rifle jamming problems, it was found the brass used was too soft and the case hardness was increased.

[Robinhood]

Let me quote myself....

Loose primer pockets. Often people say that over pressure loosens primer pockets. While that may be true, more often the culprit is an oversized chamber combined with high pressure loads. If a reamer is out of spec or the tailstock center-line is off just a few thousands of an ", it can cause the base of the chamber to be cut OS. Over polishing could be the culprit also.

[bigedp51]

Let me quote myself....

I collected milsurp rifles that had large military chambers and if you wanted longer case life you used harder and better quality brass. With commercial rifles and cases you still have plus and minus tolerances for the chamber and case diameter. Bottom line the brass case has elastic limits and this varies between brands of ammo. And if you apply too much pressure to that brand case the brass will not spring back.

I understand what you are saying, if you have a fat chamber and skinny brass the case can expand in diameter more. But the quality of the case and its construction also plays a big part in case life.

[hardnosestreetcop]

On the vast majority of rifle cartridges the very base of the case is not supported with the exception of rimmed cases. Meaning a over sized chamber would cause a bulge where the case does contact the chamber. "BUT" high pressure will have a effect on primer pocket life.




One of the biggest problems of short primer pocket life is the thickness of the flash hole web and the hardness of the brass. Below you can see why Federal get a bad rap for oversized primer pockets after a few reloadings

.

Below Lake City and contract ammunition for the military uses harder brass and thicker flash hole webs.





At the link below measuring the expansion just above the extractor groove doesn't tell you the chamber pressure. It just tells you how soft the brass is and the strength of the flash hole web. Meaning the same load in a Federal case would show more base expansion than a Lake City case.

Simple Trick for Monitoring Pressure of Your Rifle Reloads

http://www.hodgdonreloading.com/relo...-rifle-reloads

Ive got a Boat load of the PMC .308 Brass, it’s definitely heavier than my Winchester commercial 308 brass. Thank Ed

[darkker]

Ive got a Boat load of the PMC .308 Brass, it’s definitely heavier than my Winchester commercial 308 brass.

Cases weigh different amounts for several different reasons:
Balloon head Vs flat head, alloy difference, standard lot variance, primer size, etc. When Frankfort and Pic were testing 5.56 in the 70's over the dropped primer issue, they found a stunning variance in case weight even from the same production line; whereas the actual volume difference and corresponding pressure differences were negligible. I also pressure tested this fact out a couple years ago when testing LR Vs SP Creedmoor brass. Despite marketeering wank by Lapua, they had a significantly larger case variation in weight than the Norma. Regardless, loading identical charges by volume on both cases, produced the same pressures.

Texas,
It is true that Federal heat treats differently than "most", especially towards the case head. The faster the case obturates, the potentially more accurate the ammo. Couple that with a larger chamber and you can have issues. I've had cases that grew significantly in the extraction groove, blew primers out and was general unpleasant, running PT confirmed @45,000 psi loads.

If you want to know volume and how your burning curve will be impacted, measure volume. If you simply want to know how much winter weight you've gained, weigh the cases.

Cheers

[Robinhood]

Thanks for the good input Tex and Dark.