shoulder set-back

I am asking this here, as it seemed appropriate; how does a case shoulder get 'set back' by the primer in mild to low pressure loads when the case is formed to the chamber and headspaces properly? I was reading the "Cast bullets for Military Rifles" article, as well as the "Mouse fart loads" article, and have seen this phenomenon mentioned elsewhere. The articles also mention drilling the primer pocket flash holes out with a #28 bit to prevent this from occurring. Can someone explain this in general terms? Thanks, gents.

Don't know if it is really set back as I think of. Primer pops, pushed the case back to the bolt and the powder charge pushes the shoulder FORWARD. Opening up the flash hole is an attempt to get the powder to light right away. Head space is the tolerance allowed for the chamber OAL. Milsurps usually have loose chambers. Fire formed still isn't exact chamber dimensions.

Now that makes more sense. Wouldn't pushing the shoulder forward be desirable, in order to get a better fit? Especially in a mil-surp?

In a low pressure load the primer pops and the case is pushed forward until the shoulder or rim stops the forward travel. This also makes the primer back out.
With a low pressure load the case walls expand and grip the chamber. The pressure inside the case is never enough to make the case slip back or stretch. So the shoulder remains driven back or unexpanded. This is easy to find in the low pressure .35 Remington even with full power factory ammo.

OK, let me give some more info; I am experimenting with plinker loads in my SKS, using Red Dot, 700X, and Titewad, using a Lee 314-90-SWC, as well as Lee's 309-150-F. I have noticed some primers backing out, but not the shoulder issue. So, I was wanting some education concerning the shoulder being 'set back'. How does drilling the flash hole to a larger size affect this?

You may have the shoulder set back since you also have protruding primers. You would need a case gage or Hornady tool to measure the location of the shoulder.

The primers protrude because when they fire they cause high pressure in the primer pocket and back out. They are able to generate the high pressure because the primer explosion is forced through the restriction of the small flash hole. If the flash hole is drilled out there is very little restriction of the primer explosion so it does not push the primer back out of the primer pocket. When the primer cannot generate the pressure to back out it is also NOT pushing on the bottom of the primer pocket because the bottom of the primer pocket was drilled out. So the case does not get shoved forward and the shoulders stay where they belong. Change primers, powder charges, bullets, have oil on your cases or a number of interrelated conditions may cause the shoulder to move or not move. It will not always be a black and white condition.

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Originally Posted by Bloodman14

OK, let me give some more info; I am experimenting with plinker loads in my SKS, using Red Dot, 700X, and Titewad, using a Lee 314-90-SWC, as well as Lee's 309-150-F. I have noticed some primers backing out, but not the shoulder issue. So, I was wanting some education concerning the shoulder being 'set back'. How does drilling the flash hole to a larger size affect this?

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Note to self - do NOT used enlarged flash hole brass for standard loads! I used to shoot a lot of light loads in 30/30 thin brass (but it is rimed H.S.). Never noticed the problem even when I (mistakenly) used magnum LRP. It is the delay between primer ignition and powder ignition that is the supposed problem. IMHO I can see the primer coming out a bit but to shove the shoulder back on 308 type cases?

Since the case headspaces on the shoulder of a rimless case, I just can't see how the shoulder gets moved at all. If the shoulder does get moved forward upon firing, I would still think it would form the case better in the chamber. As noted by EDG, I tend to be a black-and-white thinker. Having a mechanical and fabricator background is interfering with my thinking, I guess. Always something new to learn.

The shoulder of the case does not get moved forward on firing.....it gets moved back.

I shoot many thousands of squib loads in various calibers but mostly in .30s. Many of these are rimless cartridges; 30-06, .308, .308 CBC, 7.65, 7.62x39 etc. The squib loads I shoot most often is a Lee 314-90-SWC-TL over 2.7 to 3.2 gr of Bullseye depending on the cartridge. Velocity is around 800 – 875 fps. I found a long time ago the shoulders do in fact get set back with light loads such as those. With many cast loads that use normal weight bullets in the 1600 to 2000 fps range there was little setback. It basically is a matter of the psi the load generates. It takes roughly 7,000 psi (depends on thickness and hardness of the brass along with how much the case needs to reach the chamber walls.

Measurements of shoulder set back or increase are easily taken with a Stoney Point tool. There have been basically the two theories regarding the cause; the firing pin blow theory and the primer theory. I ran the same tests with a fire formed case and inert primers; headspace was not changed. I then used the same fire formed case with live primers. In as little as two firings there was a measurable decrease in headspace. After five live primers the fired primer was noticeably backed out after firing. NOTE: this increase in headspace was with case taking LR primers. I never experience the problem with the .222 Rem or the 5.56 Nato.

Using #'d drills I gradually increased the flash hole diameter with a progressively larger drill. Using a different fire formed case with each larger drill and firing 5 primers I then measured the headspace before firing and after. As the size of the flash hole increased the headspace decrease lessoned. With a # 29 drill I no longer got any decrease in headspace. I dedicated five .308 cases and five 30-06 cases that were well fire formed to their respective rifles chambers and drilled the flash holes with the #29 drill. Over the next few days I fired 50 shots with each case. There was an indoor 50” range where I was stationed so it wasn’t all that bad. After the 50 firings there was negligible change in headspace with any of the five cases of each cartridge. The results of my test firmly demonstrated to me that it was the force of the primer explosion that drove the case forward and set back the shoulder. The squib load does not have the pressure to expand the case out to fit the chamber. By drilling out the flash hole the force of the explosion mostly went directly into the case as there is little rim left to contain it. Two other side benefits that were unforeseen; the extreme spread and standard deviations of the velocity readings improved and the case position sensitivity of the small charge was greatly reduced.

As a result of the above tests I dedicated fire formed cases for squib loads for each rifle in rimless cases and drill out the flash holes. I have fired them many, many times now with no further change in headspace. Besides the squib load mentioned I also use 311631 (# may be wrong but it’s the 118 gr GC 32-20 bullet) with Unique in the above cartridges loaded to 1400 fps or so for a little more powerful small game load. The flash hole drilled cases work just fine for those. I now use the flash ole drilled cases for all my rimless cartridges with squib and really light loads.

Further pressure testing in the .308W the last few years indicated that loads with a psi above 12,000 will obdurate sufficiently to prevent the primer from driving the case forward thus setting the shoulder back.

Problems associated with shoulder setback (misfires after several firings) are most common with rimless cartridges in push feed rifles. Rimmed cartridges are held to the rim headspace dimension of the rifle. In CRF rifles the case shoulder can be set back only until the extractor holds the rim and stops the case from moving forward.

The shoulder can be moved in either direction depending on what firing process produces the most force - the fired primer or the fired powder charge in the case. It is easy to see shoulders blown forward in the .303 British because the chambers are much longer than factory ammo. SAAMI standard dies produce shoulders that are located much further to the rear than the shoulder in the chamber. So it is very important to NOT push the shoulder back with .303 dies.
With low pressure rimless cases you can have the condition that will set the shoulder back. If continued this can result in case head separations.

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Originally Posted by Bloodman14

Since the case headspaces on the shoulder of a rimless case, I just can't see how the shoulder gets moved at all. If the shoulder does get moved forward upon firing, I would still think it would form the case better in the chamber. As noted by EDG, I tend to be a black-and-white thinker. Having a mechanical and fabricator background is interfering with my thinking, I guess. Always something new to learn.

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Great explanation, thank you Larry. You made that really clear and easy to understand.

Anyone with any experience with the .35 Remington and the .30-30 knows they will find a large percentage of backed out primers with full power factory loads. These backed out primers get that way from moving the shoulders backward even though the chamber pressures exceed 30,000 PSI.

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Originally Posted by Larry Gibson

Further pressure testing in the .308W the last few years indicated that loads with a psi above 12,000 will obdurate sufficiently to prevent the primer from driving the case forward thus setting the shoulder back.

Problems associated with shoulder setback (misfires after several firings) are most common with rimless cartridges in push feed rifles. Rimmed cartridges are held to the rim headspace dimension of the rifle. In CRF rifles the case shoulder can be set back only until the extractor holds the rim and stops the case from moving forward.

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EDG, the only way I can visualize a shoulder being moved back on a rimless case is if the firing pin strike can hit with enough force to shove the case further into the chamber, which would have the added effect of reducing the head diameter .001 or so. This would indicate to me that the case is always shorter than the chamber. I don't know if this makes a difference, but I only neck size my cases after the initial firing. This applies to both my 2 Enfields and my SKS. The shoulder is never moved while being processed, so should be in the same place with each subsequent firing. Using the loads described earlier should have no effect on shoulder location. It looks like I need to get a case gauge and do some checking.

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Originally Posted by EDG

Anyone with any experience with the .35 Remington and the .30-30 knows they will find a large percentage of backed out primers with full power factory loads. These backed out primers get that way from moving the shoulders backward even though the chamber pressures exceed 30,000 PSI.

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Would this indicate some excess space between the bolt face and the head of the case when chambered? How would the primer have any room to move?

If you examine the drawings for SAAMI standard chambers and ammo both have tolerances.
The shortest acceptable ammo in the longest acceptable chamber can result in the ammo being .010 shorter than the chamber.
It is pretty routine for new factory ammo to be .004 to .008 shorter than a chamber.
Ammo is rarely a perfect fit in any chamber unless the handloader makes it that way.
When the primer fires it can easily push the case forward if there is a gap at the shoulder.

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Originally Posted by Bloodman14

Would this indicate some excess space between the bolt face and the head of the case when chambered? How would the primer have any room to move?

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The force of the LR primer explosion in the primer pocket pushes the case forward in the chamber. That resizes the case just like when you push the case into a sizing die. Eccept the case is already sized so the chamber shoulder sets the case shoulder back.

I get that the pressure from the primer explosion against the bottom of the primer pocket will create a force that pushes the primer backwards and the case forwards. But, if the primer does not move in the pocket, then the primer and case are a single system and the case will not move forward. If headspace allows, the case will move backwards as the products of combustion are pushed through the flash hole.

If firing a low/no powder charge results in a backed out primer, do you think that when you fire a full power load that the primer first backs out when fired and is then reseated when the case head is pushed back against the breech face by the main powder charge?

I ask only because I've always assumed that the primer explosion is not of sufficient strength to cause a primer to move relative to the case.

Jimb I believe that in most circumstances the primer WILL move in the pocket.

Take an empty brass of any caliber, prime it, put it in a gun and shoot it.

Primer will be set out some from where it was.

It is just that with full power rounds the powder going off slams the case to the rear resetting the primer against the bolt face. But with light loads I can see by way of Larry's description why shoulders are slowly pushed back.

And I could be wrong here, but I do not believe that I am.

" I ask only because I've always assumed that the primer explosion is not of sufficient strength to cause a primer to move relative to the case."

That assumption is incorrect. How much force does it take to seat a primer.....really not much compared to pressure created by the primer explosion. Primers, especially LR primers, have sufficient strength to blow bullets out of the case and stick them in the throat, forcing cone and leades of barrels w/o any powder burn.

"But, if the primer does not move in the pocket, then the primer and case are a single system"

Another incorrect assumption. Primers, when fired, always move in the primer pocket. The exception is when the primers are crimped in as in modern milsurp ammo. When primers are seated to the bottom of the primer pocket (the correct method) they are slightly below the case head because primer pockets are a bit deeper than the height of the primer. When fired with pressures high enough to fire form the case the primer will be flush with the case . Sometimes the primer can back out slightly first then is slightly flattened as the case is expanded and pushed back against the bolt face. That is why "flattened" primers can be a false indication of excessive pressure.

The testing is conclusive; LR primers alone or with a very low pressure load in rimless cases will push the shoulder back with each successive firing which, in push feeds particularly, can lead to misfires and failure to extract.

"And I could be wrong here, but I do not believe that I am. "

Easy enough to find the answer; if you have a push feed rifle for a cartridge using LR primers test it yourself. Take a sized case and just prime it. Shoot it in your rifle then deprime, reprime and shoot again. Repeat and see how many times it takes before you have a failure to fire and/or a failure to extract.

Try firing primed cases in a revolver. Most lock up solid due to primer movement.

Thank You Larry Gibson!

Solving the problem with CB loads is easy. Seat the boolit so that it contacts the lands. Then the case cannot move forward and will fireform to fit the chamber.

If you see primers backing out on RIMMED cases, you have excess headspace. Pretty common in old, hard used 94s.

Ditto belted cartridges which are all factory loaded with grossly undersized bodies and shoulders. Cases stretch all to heck, life is short but primers don't back out unless headspace is excessive.

All primers back out and are reseated by case pressure. Even with zero headspace the primer that was below flush become flush after firing.

So it appears the primary problem is with LP that have a small flash hole and large area under the primer. The tolerance of any chamber/ammo will allow the primer to move relative to the case. The primer MAY get smashed back into the pocket. Repeated light firings in the same case will show the accumulated set back. OK. Even fast pistol powder can't generate enough pressure (for light loads) to hold the case in place. Hypothesis validated by data. I still wonder about the case being pushed back from the boolit engraving? In my case, all brass is in the same bucket and may get a high pressure load next time, so I don't worry about it.

Earlier this year I was doing some light load work with a small batch of .308Win in my Win M88. After a few loading cycles, I started to get high primers but didn't really worry about it to begin with. That is, until I actually started to get failure-to-extracts; the fired primers were visibly higher than they used to be and the case rim appeared to be actually slipping past the extractor, setting the case further into the chamber. I set up false shoulders (w/ an 8mm expander) to get the cases under control, then did the #29 drill bit routine. Problem solved & primers are staying flush now - thanks LG. :)