Another pressure question.

[ShadeTree]

Dougfir's question about pressure and velocity got me to thinking about another pressure question that I've sometimes thought about but never really figured out.

Are pressure specs only for the cartridge design, or can it also be for the action and chamber they were designed for?

In other words take a low pressure cartridge like the 30-30 Winchester designed at the time solely for a lever gun, and by far the predominant action that round is run in yet today. If that cartridge is put into a stronger locking action like a bolt gun can it be run at higher pressures or is the cartridge case design such that it wont safely stand working pressures higher than the 38,000 cup, or 42,000 psi that SAAMI established for it?

 

[Polaris]

Some of both. A good example is the .45-70 govt. This round is factory loaded to BP "safe" pressures, but can routinely and safely be loaded much higher in modern actions such as the Ruger #1 as the original case design and construction is inherently strong enough.

The .30-30 can be souped up some as well, as is done with the .218 Bee and .219 Donaldson Wasp, but I don't think you'd want to try and bottle up 60k PSI in some sort of wildcat.

I know some of later BP cartridges are very weak, and have never been upgraded to stronger cases as a relatively thin walled case is necessary to properly obturate with their low operating pressures.

In some cases, the action dictates the operating pressures. This is the case with .35 Rem and .300 Sav, as well as the 6.5x55 Swedish/6.5 Skan. In the former 2, weaker actions dictate lower maximum pressures, but in a strong bolt action either can be loaded to higher pressures. With the 6.5 Swede, the strength of original Mauser 96 and Krag actions dictated a lower operating pressure, but higher pressure load data is available for modern M98 and Sako actions.

 

[Silent Sam]

More the action than the brass. While some cartridge designs are more prone to stretching etc. than others ultimately it is the action that holds pressure. The brass is more or less a gasket. See "Ruger Only" loads...

 

[Gunner46]

Can't say any more than what Polaris & Sam have already. I'd say action strength first, case strength second and powder selection next......

 

[Nimrod84]

Can't say any more than what Polaris & Sam have already./quote]

+1

Will go a little different direction and say that in general, the SAMMI pressure spec is generally specific to the cartridge not the action. Ammo is held to that spec for use in any and all actions - if you want to build a lever action 460 Weatherby, you are welcome to do so but it needs to be able to handle the cartridge.

Supposedly, Europeans tend to do it a bit differently as they have standard lower pressure loads for older rifles and +p loads for modern rifles. Might just be internet scuttlebutt though... Wikipedia's page on the 6.5x55 has: "Finnish powder manufacturer Vihtavuori warns modern 380.00 MPa (55,114 psi) Pmax piezo pressure loadings should never be used in the Krag–Jørgensen or Swedish Mauser or similar older rifles."

 

[ShadeTree]

Okay so pressure specs are typically based on the action it was designed for, or at least in the examples of older designed cartridges.

That brings me to another question then. I reload for my Dad's old model 71 in 348. Saami specs for the 348 shows a maximum average service pressure of 40,000 cup.

When looking at modern loading data, UNLIKE typical cartridges like the 30-06 that has some powder's max loadings right at or even a couple hundred over the 06's rating of 60,000 PSI, most 348 max load data is well under Saami specs at somewhere around 36,000 cup or under. I'm loaded 6/10 of a grain under max with a powder that shows 35,800 at max. I typically would not load close to max with an old gun but if the max powder charge is well under rated pressures and I'm below that, I shouldn't be stressing that action, correct?

 

[Nimrod84]

Shade Tree,

I'll leave a direct answer to others who know the 71 action better than me.

Having wrote the above, I would feel feel pretty comfortable using current load data with current production powders and primers up to the max loading. I would start relatively low and work up to the max, just to make sure no surprises show up in full force.

However, the model 71 may warrant downloading due to availability and pricing of parts / wear items.

Regarding your loads being about 4,000 CUP under max. From talking with a sales and a tech rep from two different companies, it was mentioned that a lot of the down-listed loads are due to variables that aren't reported in the manuals. They might see inconsistent pressures, large powder burn rate changes with compression or space in the cartridge or changes from primer substitution, user variables such as crimping, etc. that force the techs to keep the loads milder than they would otherwise have to be.

FYI - I deal with a similar situation with my 280 Rem's, they were designed for lever actions / semi's and pressures are held to 50,000 CUP or about 60,000 psi. Realistically the round can be pushed to the same 65,000 psi of the 270 Win, but manuals often list loads in the 48,000-49,000 CUP or roughly 57,000 psi range. Aforementioned reps indicated that the hotter I run the 280's the better they would be, main issue I recall is that the loose space (air gap) in the cartridge allows powder to be on the bullet or on the primer and thus you get weird stuff. H414 gave me a lot of problems - hang fires near the start loads, but smoothed out at higher charges / pressures...

 

[ShadeTree]

Nimrod 84, I typically don't run loads with the same components as the book listings, but in this case I just happen to be using everything exact. Same case, same primer, same bullet, same barrel length, and same COAL as the listed load using that powder.

H Varget with a starting load of 53 gr's and a max charge of 58 gr's with a listed velocity of 2605 at max charge and 35,800 Cup pressure. (This is off of Hodgdon's website as I don't have a reloading book showing the 348.)

I loaded up 10 rounds in .5 increments starting at 53.5 going up to 58 and with careful inspection after each shot I didn't see any pressure signs or feel any difference in effort or tightness when ejecting any round. I then loaded up 3 rounds to verify speeds with charge weights of 57.3, 57.5, and 57.7. They chronographed at 2533, 2556, and 2601 respectively. I've since loaded and shot roughly 10 rounds through it loaded at 57.4 grains. Didn't chronograph that but I figure it's right handy 2550. I see no issues and by book measurements I should be more than safe, but by the same token I'd gladly give up 100fps if I thought I was stressing the old girl any.

 

[PJGunner]

Well, as someone said the brass case is just a gasket to keep the gas contained but construction of the case also comes into play

Good example is the 30-30 which is loaded to relatively modest pressures. Ken Water did an excellent report in Pet Loads on the 30-30. He tried loading for "hotter" rounds using a solid Winchester M54 bolt action. The end result he could not get much more from the cartridge case as it was too thin for the extra higher pressure.

We all know some cartridges are not loaded to their full potential because of weaker actioned guns ot use in pump, lever and semi-auto firearms. Again a prime example is the 7x57 Mauser due to originally being chamber in rolling block rifle and the weaker 1893,1895 and 1896 Mausers. I've seen a couple of the latter so they do exist although mainly in the 6.5x55, another cartridge kept to lower pressure levels. Another problem with the 7x57 is they have been chambered with varying throat dimensions creating pressure problems from those with shorter than SAAMI/CIP throats. Yet we all know that cartridge can be loaded to a much higher pressure level in a modern rifle such as the Winchester M70, Remington M700 and late versions of the M98 Mauser.

The one thing that I think is the most relevant though is the cartridge case. It has to withstand the rapid pressure rise when fired and "hold the fort" till pressure has dropped well past being a problem. That brass case lets go and you're in trouble. Then it's the quality and design of the particular firearm that has the final say on your body parts.
Paul B.

 

[Nimrod84]

Shade Tree,

I would be real comfortable up to the 58 grains of Varget and a 200 grain jacketed bullet. Main thing in my mind is that I don't know the peculiarities of the beautiful Model 71; what wears out quickly or what gets stressed.

I make a lot of component substitutions, but I like (pretty much need) to see a component used in a similar load before I do so. Ends up being similar, to averaging out different max loads from different manuals, but I'm doing it with components.

PJ Gunner, yup. Brass is often the weak link. A number of semi and bolt action rifles after WWI and WWII were tested to destruction, under the supervision of Major General Hatcher. They used overloaded or wrong sized cartridges (8x57 for example in the m1903 rifle). A number of times a rifle would survive, but the brass cases would effectively melt / flow into the bolts and fused everything together.

In dealing with the early brittle m1903's that might not have been heat treated properly, Hatcher came up with the bolt gas relief hole. The idea being that you want the case to fail and release the pressure in a controlled manner, before the action, barrel, stock, etc. catastrophically fails.

There is a lot of good info from Hatcher on rifles and cartridge design / dynamics.

 

[ozarkpugs]

Dougfir's question about pressure and velocity got me to thinking about another pressure question that I've sometimes thought about but never really figured out.

Are pressure specs only for the cartridge design, or can it also be for the action and chamber they were designed for?

In other words take a low pressure cartridge like the 30-30 Winchester designed at the time solely for a lever gun, and by far the predominant action that round is run in yet today. If that cartridge is put into a stronger locking action like a bolt gun can it be run at higher pressures or is the cartridge case design such that it wont safely stand working pressures higher than the 38,000 cup, or 42,000 psi that SAAMI established for it?

If the brass case fit the chamber perfect and was supported 100 % then case strength / thickness would not be an issue but they don't fit perfectly and have to stretch to conform to the chamber and then relax to be removed plus the primer pocket area has to be able to hold up to pressure with out expanding and loosing primer . During the expanding case gets thinner and pressure will rupture it .

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