Shoulder Set Back

[WineMan]

I read about the primer blast setting back shoulders in rimless cartridges with light loads. What is this phenomenon and why are rimmed cases not affected by this? How light is a light load?

Reading Hatcher's Notebook he did some work on increasing headspace and felt that as long as the extractor holds the round in the chamber there is little problem with excessive headspace in rimless cartridges.

Ed Harris recommends enlarging primer flash holes to cure the set back but to be sure that these are NEVER used for full charge loads. Not something I want to try.

Thanks for the help.


Wineman

[Larry Gibson]

WineMan

I read about the primer blast setting back shoulders in rimless cartridges with light loads. What is this phenomenon and why are rimmed cases not affected by this? How light is a light load?

Don't know that it is a "phenomenon" but it does happen, usually with very light "gallery" or "cat' sneeze" loads. An example is a 90 gr SWC cast bullet over3.2 gr of Bullseye in a '06 case. The force of the primer expolsian drived the case forward with each firing and the shoulder is sized back. Usually takes severlafiings before headspaceproblems are created. Rimmed cases are not effected as the case headspace on the rimm and is not driven farther into the chamber to set the shoulder back.

Reading Hatcher's Notebook he did some work on increasing headspace and felt that as long as the extractor holds the round in the chamber there is little problem with excessive headspace in rimless cartridges.

Hatcxhers shooting was mostly done with the M1903 and other rifles with controlled feed extractors of which the rim of the case slipped up under on feeding. The case is held back by the extractor and "excessive headspace" of the case will only be as much as the extractor allows. Most firing pin extrusion is such that misfires are not a problem thus there usually isn't a problem with controlled feed rifles. Push feeds are anothe case entirely and there the problem surfaces as when the cartridge headspace becomes excesive the case may not snap under the extracor and misfires will occur.

Ed Harris recommends enlarging primer flash holes to cure the set back but to be sure that these are NEVER used for full charge loads. Not something I want to try.

Ed Harris is correct; I use a #28 or 30 drill to drill out flash holes on dedicated well fired formed cases. I have however worked up to "maximum medium" power loads using medium burning rate powders (175 gr cast bullets to 2200 fps in .308 Win with 4895) without any apparent problems using cases with the flash holes drilled out. While I've found that to be a safe practice one must be careful to not use such cases for full powered jacket bullet loads.

Thanks for the help.

Your welcome.


Larry Gibson

[405]

Yep know it pretty well... if it is the same as you're asking about.
It is NOT the topic of interest that 35 Whelen fans want to hear though :mrgreen:
I did quite a bit of testing and fireforming in a 35 Whelen a few years ago.
The chamber was headspaced "right in the middle"- which is standard for most the regular 30-06 based rounds.

Most folks think that it is the firing pin fall that drives the case forward just at ignition. Then the pressure of the load forces the case back against the bolt face. The reason for this thinking is that repeated dry firing on a spent case will slowly force the case shoulder on into the shoulder of the chamber.... leading to an apparent headspace issue. BUT, the force of the firing pin fall is small compared to the force exerted by the explosive in the primer. The primer detonates and forces itself rearward while forcing the case forward. An equal and opposite kinda thing. This happens just a very small split second before the powder charge gets to high pressure. The case is still forward, the pressure rises, the case walls cling to the chamber walls and the head portion of the case is pushed back into the bolt face. This process tends to stretch cases just foreward of the web and can cause case head separation... over time. That's the "shiny ring" all load manual say to look for just ahead of the case head.

Now, with lower pressure loads say less than 20-30K psi or like a lot of cast bullet loads the primer fires, the case is forced forward, the pressure rises, the case clings to the chamber walls, the strength of the brass is not exceeded, the pressure drops, the primer stays protruded from the case head.... and the shooter thinks there is a big headspace problem. There may not be a big headspace problem with the gun but the cartridge design itself may not lend to an absolutely positive headspace control. The amount of shoulder left after increasing a 30-06 neck to the 35 Whelen is fairly small. Add to this small, datum ring of headspace control .... the shallow slope of the 30-06 family only 17.5 degrees of shoulder angle. There is a reason that no factory rounds were offered in say a 9.3-06 or 375-06 or the worst of all the 400-06!!! Some have wondered all along why Remington didn't go straight to the 35 Whelen AI instead of the regular 35 Whelen in their factory offerings. This headspace issue was known from the get go. The Ackley Improved version of the 35 Whelen pretty much cures this "hidden" headspace problem. It increases the size of the datum ring and it sharpens the shoulder.

Using the 35 Whelen as an example in full pressure loads or factory loads, the stretching that takes place just forward of the head does happen but there is no primer sticking out after firing. The full pressure load simply stretches the case back against the bolt face and re-seats the primer.... so the round may look normal after firing.

Now the issue of whether or not the extractor can provide enough headspace control to prevent stretching or other problems associated with excess headspace????? I guess if that were the case there wouldn't be a need for headspace control by the rim, the shoulder or a belt :mrgreen:
The extractor may provide enough resistance for the firing pin to detonate the primer but I wouldn't think the extractor should be considered the primary source for headspace control.

In the case of rimmed cartridges I don't think the phenomenon of the backing out of the primer would be at issue because of the rim..... the rim by design is such a positive cartridge STOP providing very positive headspace control. On the other hand the gun may be a little sloppy in headspace control because of bolt compression or set back.... like in some older lever guns that have been hammered by high pressure loads.... then, yes, the backed-out-primer is a give away about sloppy headspace.... but not because of the nature of the rimmed cartridge.

In those cartridges like the Whelen, a work around is to have the chamber reamed to minimum headspace when the gun is being built or just go straight to the AI version. With the minimum reamed headspace solution it's even better to go .001-.002" short where when chambering factory sized new brass... the shoulder contact can be felt- kind of a squashy feel just as the bolt is closing down. That way the brass is not stretched on the very first firing and the longest case life can be expected. For those just shooting factory Whelen then "out of sight, out of mind" No problems
Sorry about the windy post- but seemed necessary to adequately explain the backed-out-primer phenomenon as I understand it.

[NVcurmudgeon]

This is a perrenial subject of interest. I have been shooting moderate CB loads in rimless rifle cartridges for 35 years. Rimless cartridges loaded for are 7 X 57 Mauser,.30 Remington, .308, .30/06, 7.65 Mauser, 8 X 57 Mauser, and .35 Whelen. Nearly all loads were in the 1200-1800 fps range. Only the .35 Whelen cases ever shortened head to shoulder length enough to cause misfires, and that after ten firings. Changing form partial FL sizing to a true neck sizing die eliminated that problem. I now have cases that have been fired more than twenty times with no head to shoulder shortening.

[jhrosier]

I am shooting moderate CB loads in both the 8x58RD and 10.4x38 Swiss cartridges.
My brass is formed from cases with thin rims and is sized to seat on the shoulder.
I load the 8x58RD with Unique and try to stay under 28Kpsi, and the 10.4 with Hodgdon's Triple 7 powder, a BP substitute.
I was experiencing primer setback, as well as inconsistent ignition with both of these cartridges.
I substituted Winchester Large Pistol Magnum primers for the large rifle primers in both of these cartridges and the primers no longer set back and the velocities became much more consistent. Accuracy improved as well.
The edges of the primers and the firing pin indent are slightly rounded, indicating reasonable pressures.

I would not suggest using pistol primers in any load that produces more pressure than the magnum pistols that these are designed for.

I have no way of proving this, but I think that the high pressure of the rifle primers was causing the bullet to start to move before the powder charge was properly ignited. This was only happening some of the time and if I fired 20 or more shots at the same 100 yard target, I sometimes would get 2 or 3 distinct groups of shots.

Jack

[NVcurmudgeon]

Having solved the shoulder setback problem in my .35 Whelen with moderate cast boolit loads, one might ask, "Yeah, but how good is it with full power loads?" I load jacketed bullets with RL15 powder exclusively because it didn't seem worth the effort to mess with immediate success. The late Finn Aagard had an article in the American Rifleman for which he used RL15 in his Whelen. Using his recommended loads I have shot many three-shot groups with 250 gr. Nosler Partition, 220 gr. Speer flat point, and 250 gr. Speer spitzer bullets. With a load not quite up to Aagard's maximum, the Oehler 33 reads just over 2600 fps. with either 250 gr. bullet. Every case is checked wth a case gauge and a pointed wire separation detector after every firing. Despite "expert" opinions to the contrary, no incipient head separations have been found yet. Cases gauge perfectly for ten neck sized firings, after which they are moved to light duty and used for CB loads only.

I wonder if the stories of .35 Whelen headspace troubles began when the cartridge was a wildcat and there was no SAAMI standard? Both the .35 Whelen and the .358 Winchester have more shoulder area than the .35 Remington, 9.3 X 57, and 9.3 X 62. I once took a prominent gunwriter and former ammunition factory expert to task over this issue. His reply was essentially that he had written that .35 caliber rimless cartridges were no good so it must be true. Can't blame the guy for selling magazines, writers have to eat, too.

[Larry Gibson]

As mentioned with moderate or heavier cast loads it is seldom a problem. The problem does happen quite frequently with "gallery" or "cat's sneeze" loads> These are very light loads which do not have enough pressure for the case to obturate and grip the chamber. It is also the reason that well fire formed case use is recommended, to lesson gas leakage around an unobturated case.

Larry Gibson

[WineMan]

It is really great when the light bulb clicks "on".

I now have a much better understanding of what is going on and how it happens.

Thanks again to all of the contributors!

Cheers

Wineman

[DonH]

I don't have a .35 Whelen but do have a .338-06. The extractor DOES NOT hold the case head against the bolt face positively enough to eliminate problems mentioned above. My rifle is a Mauser action and if any extractor would do this, it would be the Mauser type. Early on I did experience shoulder set-back and case thretching to the point of near separation. Fixing the problem was a simple process after reading a Ken Waters article. When forming cases I necked up to .35 then back down to .338 leaving a small secondary shoulder which gives a "crush fit" when closing the bolt on a chambered round. Firing forms the case to MY chamber and no more set-back or stretching. Neck or partial resizing will help keep things where they should be as already mentioned.
This is all a different issue from the "light load" thing discussed above

[Hipshot]

Seating the bullet into the rifleing should cure any headspace problems with light CB loads!
Rimmed cartridges usually don't have these problems due to the cartridge has a rim and can only pushed so far foward! It is ALWAYS better to have a cartridge headspace on the shoulder no matter if it is rimless, has a rim or has a belt and again have the cast bullet touching the rifleing.

Hipshot

[Ricochet]

Less opportunity for the cartridge to wobble around in the chamber and start the boolit into the bore crooked.

[Hipshot]

The headspace is created by the firing pin blow on the primer which wants to move the case foward, thus pushing the shoulder back-------crush fit the brass in the chamber and have the bullet touching the rifleing (NO PLAY -NO HEADSPACE !!!

Hipshot