Sml Primer Pocket Allows Higher Pressure Than Lrg?

[BumbleBug]

I recently built a Ruger SS in 6.8mm SPC. A very interesting cartridge & lots of info because of AR aficionados. Even a whole website is dedicated to it.

Early 6.8 brass from Remington & Hornady had large primer pockets. It seems like shooters want to avoid these in favor of small pocket brass. The stated reason being that with the smaller hole in the case head the case will take more pressure. Is this true? Do any test confirm this?

[ShootistPRS]

While I am pretty sure that brass of a given outside diameter with a smaller hole will take more internal pressure because there is more brass around it, I am also pretty sure that the pressure to to expand that part of a case is well over the maximum recommended pressure for any round in any commercial rifle.

It's one of those things that makes people feel better. There is nothing wrong with feeling better, it builds confidence so it is good. It can be proven mathematically but in real life, under the working pressures of the round, the difference is not relative to the guns being used.

[disseminator]

It's about case head strength. This is why the 454 Casull uses Small Rifle primers instead of large pistol magnums.

More pressure = more force against the case head.

So smaller primer allows for more brass in the case head.

[Unclenick]

Yes and no. The fact there is more metal to stretch does add resistance to lateral plastic flow (diameter expansion). However, unlike large rifle primers, which seem to be pretty uniformly stamped from 0.027" brass sheet, small rifle primers come in a range of cup wall thicknesses, so you have the problem that some can blow out and pit your bolt face more easily than a large primer is likely to do.

If you do as board member F. Guffey does, and measure your chambers by transfer measurement and resize based on not moving the shoulder back too far from the length of your actual chamber headspace, you will have less impact expansion of the head to begin with. You will still have pressure-caused flow, but avoiding the initial slam helps. (Mr. Guffey invented a nice tool that measures flash hole expansion as the earliest sign of a case head starting to get too wide, and he was kind enough to send me one. Unfortunately, I don't think he has it in current production.) The problem, in part, is that every maker seems to use slightly different alloys and levels of work hardening at the head. Because they sometimes outsource brass forming among one another (see the current Norma manual for how unexpectedly frequent this is; Norma has made Federal and RWS and Remington cases at times), you also can't just rely on a certain brand to be a certain way always. You have to check each lot. When you do, you'll find some lots handle pressure better than others, while still others let you get more reloads before they start to split. There always seem to be trade-offs.

There is a second, less commonly referenced consideration: Small primers contribute less to the total start pressure. This means primer-to-primer variation is lower when you use a small base primer, and still lower when you have the smaller European small primer flash holes. if the powder you use fills the case well, so there isn't significant extra space in need of primer pressurization, and if the powder ignites easily, like a single-base stick powder, then there is potential for improved velocity and pressure curve consistency. I remember about 15 or 20 years ago RWS produced and unusually mild lot of small rifle primers and the benchrest community went into panic buying mode over them, trying to get all they could. They found it helped produce smaller groups in their circumstances. One could argue about why, but I just wanted to throw it out there so it can be viewed as one more variable to experiment with in brass that has both primer pocket sizes available.

[Slamfire]

Early 6.8 brass from Remington & Hornady had large primer pockets. It seems like shooters want to avoid these in favor of small pocket brass. The stated reason being that with the smaller hole in the case head the case will take more pressure. Is this true? Do any test confirm this?

If that was true, then rimmed cartridges ought to be able to take more pressure than non rimmed cases. I don't believe that either.

Maybe they went for small primer brass for the same reason we are now seeing 45 ACP primed with small pistol primers.

[Metal god]

The question I have is why did the 6.8 originally use LR primers ? Is it a low pressure cartridge ? A quick search seems to show it is a tad low at around 50k psi ish , nothing like 62k of the 5.56 . Which leads me to my next question . Are reloaders trying to push loads past the 53k psi range of the 6.8 spc ? If there are those trying to push it to 60k+ then the smaller primer may help strengthen the case head ????

I don't know just thinking out loud here ?

[Unclenick]

MG, you've got some apples and oranges in comparison there. The 62 kpsi 5.56 number comes from the European channel transducer spec used by NATO and the CIP, which is 4300 bar or 62,366 psi. On our conformal transducers it reads about 55,000 psi with a 55 grain bullet and about 58,000 with a 62 grain bullet when the channel transducer gives both loads that same 4300 bar reading. So you want to compare that 62 kpsi number to the European conformal transducer peak pressure rating for the 6.8 Rem SPC, which is 4050 bar, or 58,740 psi.


Slamfire has offered up an interesting proposition. Thicker metal should take more effort to stretch. I currently do not load any rimmed rifle cartridges other than 45-70, which is definitely at the low end of pressures. If someone has a safe but stout load for an Enfield or other strong action in .303 British or 5×57R, it would be interesting to learn how long it takes for primer pockets to stretch in cases with the rims turned off (cleaning rod ejection, alas) as compared to those left intact from the same lot.

[Metal god]

MG, you've got some apples and oranges in comparison there. The 62 kpsi 5.56 number comes from the European channel transducer spec used by NATO and the CIP, which is 4300 bar or 62,366 psi. On our conformal transducers it reads about 55,000 psi with a 55 grain bullet and about 58,000 with a 62 grain bullet when the channel transducer gives both loads that same 4300 bar reading. So you want to compare that 62 kpsi number to the European conformal transducer peak pressure rating for the 6.8 Rem SPC, which is 4050 bar, or 58,740 psi.

You know , while I was writing my post . All that above was going through my mind and I was thinking the numbers were actually the same and yet I kept writing .

That does bring up a question that the answer may be obvious but I'll ask to be sure . You bring up the differences in the "European" channel transducer and the conformal transducers . Is it reasonable to assume the LC plant here in the US uses the European channel transducer when checking there NATO spec ammo ?? I assume they need to check pressure every time they get a new lot of powder in order to be sure it matches Spec ???? At least I believe they do . I've heard they don't load a specific charge but rather to a specific pressure ???

[Unclenick]

They were using Kistler transducers (the European ones) for a time after they first dropped the M11 copper crusher. But in 2012, to be able to extend the ability to resupply ammo to commercial manufacturers without resorting to different sets of test gear, ATK Orbital made the Lake City plant move over to the SAAMI type conformal transducers. So, now you see the Europeans load M193 to the EVPAT 5.56 4300 bar standard and Lake City to the SCATP 5.56 55,000 psi standard. But when you swap the same loads between the two types of measuring equipment, they read correctly in both. The difference is a little like having one scale that reads pounds and another that reads kilograms. They'll both be right, but the digits displayed will be different numbers.