View Full Version : Pressure Specifications of actions and brass
July 21, 2009, 06:12 AM
I've just managed to surprise myself over how much difference brass can make to maximum safe load -- I've found 5 grains difference between loads to a 270 going from a relatively inexpenisve brass to lapua in my 270 win.
And I still don't have any pressure signs.
This leads me to wonder -- if brass makes so much difference, how much difference does the strength of the action make?
I've looked through cartridges of the world, and see presure ratings from 40 000 psi to 65 000 psi for available cartridges according to the load data. Some wide variations in the cartridges marketed in the same action lengths.
So presumably a manufacturers actions are spec'd for the maximum pressure cartridge they manufacture that load for.
Is it then true that if the action is spec'd for a higher pressure case than the one you are using in the action -- it is likely to be safe to load it to higher pressures than those standard for the cartridge you are using.
An example to clear up what I mean. 270 win is a fairly high intensity case, I'm guessing -- in the 30-06 action. However 7 X 57 is a pretty ancient but excellent case sometimes sold in modern rifles in the same action. Presumably in a modern rifle, what would have been customary pressures for the original
7X57 case can be exceeded?
Now I am just guessing there that the pressure specs for 270 are higher than those for the 7X57 -- to illustrate the point.
Is this correct? I'd bet it is.
So which are the strongest actions on the market ? How do you know what their specs are?
Are "magnum" actions specified to higher pressures? What pressure?
What about custom actions such as stiller actions, say the predator, or the tactical, or their benchrest offerings. Are they over-spec'd as far as pressure tolerance or are they not really capable of tolerating any more pressure than
production offerings? Has anyone had experience loading for these types of action? What have you found in terms of pressure tolerance?
Looking forward to hearing from you!
July 21, 2009, 08:12 AM
And I still don't have any pressure signs.
But you still have the pressure. Harder brass just doesn't show it as much.
There was a fad at one time to build up a Mauser sporter in .280 Rem and form brass out of Winchester .270 cases because that was the hardest and strongest available. This let them overload the .280 without "pressure signs" instead of going to the trouble to set the Mauser action up for an Improved or Magnum cartridge. I don't know what the service life of one of those pretty 1909 Mausers would be, but I doubt it was as long as they would last with standard loads.
Rocky Gibbs would furnish a list of lot numbers of military brass he recommended for his blown out and overloaded cartridges.
Remington and Weatherby kind of paved the way bragging about the overpressure strength of their actions, but it does not much affect the shooting pressures once you get up to the current level of 65,000 psi.
July 21, 2009, 09:21 AM
"So presumably a manufacturers actions are spec'd for the maximum pressure cartridge they manufacture that load for".
SAAMI operating pressures for standard and magnum factory loads range from 45k to 60k. For safety and legal considerations, actions are spec'd for much higher pressure. I'm not aware of any difference in standard or magnum actions.
July 21, 2009, 12:02 PM
Now I am just guessing there that the pressure specs for 270 are higher than those for the 7X57 You are correct, the 7X57 operates at lower pressures than the 270.
Are "magnum" actions specified to higher pressures?Typically, "Magnum" actions refers to the length or diameter of the action. In order to accomodate longer cartridges (such as 375 H&H) or larger diameter cartridges (like the 505 Gibbs), some manufacturers make a so-called "magnum" action. You can make the action stronger by increasing the amount of metal in the stress areas, but this has a way of making the actions extremely heavy. Besides, too much pressure can be detrimental to accuracy.
What about custom actions such as stiller actions, say the predator, or the tactical, or their benchrest offerings. Are they over-spec'd as far as pressure tolerance or are they not really capable of tolerating any more pressure than production offerings? That would be a question best directed at the folks who designed those actions. I suspect that they are not capable of handling any more pressure than a factory production action. Most high-pressure application machinery has a 50%-100% overpressure safety margin, and I suspect that rifle actions are the same. But the weak link in most firearms is the brass, which starts to extrude at about 70,000 psi and flows readily above 75,000 psi. I have seen actions that were overloaded and effectivley brazed shut by people's handloads, although I doubt that the action's steel itself was seriously affected. So just stick within the pressure limits and you'll be fine.
July 21, 2009, 02:54 PM
Primers usually start to fail soon after brass flow occurs into ejectors.
The BR guys run up to the edge, and there is even equipment available to allow swaging primer pockets that have expanded to allow further use of the brass.
O'Conner Rifle products used to sell 'steel head' cases.
They had a steel head (.308 size) for about the last 1/2 inch of the case, and a brass front that screwed on and was formed.
If you run the math most rifles have a lot of margin in the locking lugs and the barrel and front of the action.
July 21, 2009, 03:41 PM
Rifles are generally proof tested to about 1/3 over maximum working pressure (about 80k psi for a .308 Winchester, for example). The rifles themselves will stand a lot more than that, but the brass is the weak point. For example, Remington tested the Model 700 with loads giving some horrendous pressures with no failures, but the bolt locked up and had to be hammered open, not the kind of thing anyone wants in the middle of a deer hunt.
Note that overall pressure in pounds per square inch (psi, the usual pressure figure given for cartridges) is only part of the picture. The internal base of a .30-'06 case is roughly .10 square inches, so the pressure pushing back on the case head and locking lugs is some 5000 pounds. For an understanding, think of a rifle sitting on the ground with the barrel straight up, and with an (imaginary) rod down the barrel resting on the bolt face. On the rod is an imaginary platform on to which a Ford 150 Super Cap pickup truck is dropped every so often. That is what happens when a rifle is fired.
July 22, 2009, 06:37 AM
So it looks like any decent action will give you stacks of leeway, and the thing to do to get loads safer at slightly higher pressure is to get decent brass - which I have done.
Th five grains more, with no head expansion seems a good thing. Apart from the fact it seems to make a bit more noise!
I'd like to use up the 270 barrel, and eventually try a 280 AI. I know the difference is probably marginal, but it would be nice to b e able to use 100 to 175 grain bullets. A 160 driven at just under 3000 fps would be about right.
So from what you tell me, I figure I would be OK to stick with the sako 75, and form my 280 brass from the 30-06 (like I did the 270). I have some concerns that the brass may be a bit short, so I need to look into this.
Any of you know about forming 280 from 30-06?
July 22, 2009, 11:58 AM
If you're loading 5 grains over any book maximum, I'd consider that dangerous even with a long throated chamber.
280 cases are longer than '06, so you'd have to fireform, spending powder, primers and bullets. I'd spend the $40 and get some 280 brass.
July 22, 2009, 12:36 PM
Any time you form a smaller caliber case from a larger one (e.g., .270 or .25-'06 from .30-'06, make sure you either turn the outside or ream the inside of the case neck. When the neck is squeezed down, the extra brass has to go somewhere, which means the brass at the neck is thickened. Which in turn means that the room for neck expansion is reduced, which in turn means that the neck cannot expand as much as it should (if at all) and pressue skyrockets.
July 22, 2009, 12:40 PM
"The internal base of a .30-'06 case is roughly .10 square inches, so the pressure pushing back on the case head and locking lugs is some 5000 pounds."
While 5000 pounds sounds large, consider the area of the locking lugs that is loaded in shear.
This area is what is holding the bolt closed.
The 'hammer open' from high pressure is a function of the brass case, not the action itself.
All that said, unless you really know what you are doing and have the capability to monitor pressure, going over the book is pushing things.
Once actual measurements started occurring many of the Ackly Improved cartridges showed the gains exceeded pressure for the parent cartridge.
July 22, 2009, 12:50 PM
There might be exceptions, but most of the wildcatters who claim they get super performance from their advanced case shapes or loading methods, really get fantastic velocity figures the old fashioned way - they run up the pressure.
July 25, 2009, 07:52 PM
Nope, there is a missunderstanding here. I never said I went over the book range.
Moderately priced brass allowed me to get very low pressures, just in the lower quarter of the book range. This was getting more than 1/2 thou webbing/head expansion.
With the re-formed lapua brass (formed from 30-06 -- and yes all
tolerances measured and checked carefully for doughnuts) I could take the
load up to 59 gr of this powder without negative pressure signs -- that is the top of the book range. I feel there is probably more in it, but I have stopped there -- for the same reasons you have mentioned. Thing thing is a bit noisier than before, and I don't feel the need to chew the throat out of the chamber too much.
Unless I am wrong, lapua don't make brass for the 280. Which manufacturers do? I wont be spending money on the previous brand again, or anything comparable. It was false economy. The cases did not last long either, case neck uniformity was terrible (as opposed to the lapua which was perfect). I had to turn the previous brands neck to get them to the same uniformity that the lapua start out at !
Does this sound like an add for Lapua brass? Sorry about that. I love it.
July 27, 2009, 02:27 AM
The 270 Weatherby shoots at 71,000psi and doesn't have problems, that is factory ammo. Yes some actions are stronger than others. Here is an excerpt from "The Hunters Guide to Accurate Shooting" that I have:
"Roy commited himself to a new bolt-action, one designed in-house to bottle 200,000 psi, nearly 3 times as much pressure as traditional rifles had to sustain. He came up with an interrupted-thread locking system, like that used on some Newton rifles. Two years and five prototypes later, Weatherby engineer Fred Jennie helped Roy finish the new Weatherby Mark V. Barreled to .300 Weatherby and proofed with a charge of IMR 4350 that generated about 100,000 psi, the rifle held fast. Deliberately lodging a bullet in the barrel, test shooters fired a standard hunting load behind it. Both bullets exited after sending pressures off the scale. Save for a pierced primer and a slight increase in headspace, no damage resulted."
You have to be careful when you start getting to maximum pressures. Remember steel once it goes into it's plastic deformation it is not ever going to come back to it's original dimensions. and many, many bad things can happen.
If you want to see some FEA on a custom action and the pressures check out this page, it is on a BAT Action and interesting to read.
As a note, I've seen an overpressured Weatherby go boom and blow up in an ugly way too. Also, I would bet dollars to donuts that the Lapua brass is softer than the brass you previously had, and Norma is even softer yet in some calibers especially 6mmbr.
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