Custom action pressure ceiling.

Discussion in 'Rifles, Bullets, Barrels & Ballistics' started by RockyMtnMT, Mar 22, 2010.

  1. RockyMtnMT

    RockyMtnMT Official LRH Sponsor

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    Kirby had made a comment in another thread about a custom action handling more pressure than a factory action. My question is how do you tell when you are at max pressure with a custom action? Does it just take more pressure before the normal pressure signs show? If the pressure does not show the same in a custom action, how do you know when you are there? How much farther can you take a custom action? I am sure that this varies from custom to custom as well. I spoke with Defiance actions a while back, and he mentioned that normal pressure signs like heavy bolt lift would not show up in his action. I did not quiz him about it, it was not the right time.

    Would like to hear from you guys that are shooting, or building full customs.

    Steve
     
  2. LouBoyd

    LouBoyd Well-Known Member

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    Why do you want to load to the point you see "pressure signs". Many rifle designs will break parts and maybe cause injury with no pressure signs on the brass. For example a 50 Beowulf AR-15 can shear the barrel extension or bolt lugs without any pressure indication on the brass. It doesn't have a SAAMI spec, but would probably be around 33,000 psi if it did.

    There are several "maximum pressures" for an action.
    1. the pressure at which the action may burst with a single shot.
    2. The pressure at which repeated firing will cause mechanical failure.
    3. The pressure at which firing causes detectable changes in the chamber dimensions.
    That will eventually lead to #2 with enough shots.
    4. The pressure at which the locking mecanism will fail or be damaged.
    5. The pressure at which brass is damaged. Primer pockets can open. Bases can rupture, brass will stretch. The part the action plays is how it affects brass flow between the barrel and the bolt.
    6. The pressure at which the action cycling is affected (sticking bolt, difficult extraction, etc).

    So what determines each of the above?
    The metals used in the barrel, bolt, and chamber are important. Heat treating is important. Dimensions of the metal making up the receiver and bolt are important. Design of the locking mechanism is important. Presence of air gaps around the brass is important.

    Custom actions may be better or worse for strength and function than factory actions. Some custom actions are just pretty. Some are just cheap. It depends on the skill and care of the designers, builders, and the metal suppliers. Failure of an action is always at the "weakest" point, but that can vary with metal imperfections or heat treating variations, or machining errors. Otherwise the designer has determined where the weak points are.
    All actions are designed with safety margins, often by a factor of 3 or 4 times the working pressure. A flaw in the metal can drop that dramatically. How many manufactures micro x-ray their actions or magnaflux them? How many even proof test? Proof tests take the action near it's elastic limit which is considerably higher than the SAAMI spec, not quite to the level which would begin to give permanent deformation and far from the expected burst pressure. There is no way to non-destructively test burst pressure with certainty.

    Not all action failures are caused by pressure. Some of the worst shooter injuries are from actions which can possibly fire when out of battery where the bolt is thrown into the the shooters face or shoulder.

    Most cartridges can be miss-loaded in a way that will at least damage if not blow up the action they're chambered for. There are good reasons why SAAMI specs are what they are. Sure, there a rifles which can safely shoot a 45-70 cartridge at twice its SAAMI spec pressure, but what if someone puts a cartridge loaded like that into an old Springfield Trapdoor. They will likely be severely injured.

    We trust our lives frequently to small pieces of metal made on assembly lines of companies both foreign and domestic and think nothing of it. Mostly they're in vehicles, not guns. Consider what can happen if a front axle shears while your going around a curve on a two lane road with heavy oncoming traffic or on a mountain road if the wheel hits a rock or pothole.

    I never intentionally load any cartridge over 90% of its SAAMI spec pressure. I've been shooting for about 55 years and hand loading for 30. I've never damaged an action or had a stuck bolt. If I need (or want) more energy than a given cartridge can provide within it's SAAM limits I'll shoot a larger cartridge instead.
     

  3. jmden

    jmden Well-Known Member

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    All very good points and well taken, but does it answer Rocky's question? What about folks that are shooting wildcats?

    I've got a vested interest in the question myself with a wildcat on a custom action coming. Thanks for asking it, RockMtnMT.
     
  4. Bart B

    Bart B Well-Known Member

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    Cartridge brass starts to extrude at about 65,000 to 70,000 cup. I think that's the pressure limit one should use regardless of what the action strength is. There's some custom actions that'll hold more pressure than Winchester 70's or Remington 700's will, but why load 'em hotter than what's safe for the brass case?
     
  5. Mikecr

    Mikecr Well-Known Member

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    Yeah, brass won't let you go there in the long run.
    I'm with Lou. If I needed more than 55-60Kpsi with a cartridge to meet my goals, then I would move up in capacity.
    Just no reason to run very high pressures.
     
  6. RockyMtnMT

    RockyMtnMT Official LRH Sponsor

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    The point of the question was not to find out how far a sami cartridge can be pushed beyond sami when in a custom action. It is in particular for wildcat cartridges that have no load data. The point is to not go too far in load development. Wildcats on Lapua cases present more of a problem in that the Lapua case will take more pressure before showing it than other brass. So it seems to me that a high quality custom action coupled with Lapua brass could be a recipe for disaster.

    Steve
     
  7. Fitch

    Fitch Well-Known Member

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    +2. I'm right there with ya. There is no magic. Physics isn't negotiable. Reality is independent of the observer, at least with Newtonian mechanics which is what we are dealing with.

    The problem with pushing the limits is that there is no practical way to know what limits are being pushed and how hard they are being pushed. To an experienced test engineer, like myself, the whole reloading and internal ballistic process is remarkably devoid of measured feedback related to stress and pressure. There is no reliable measurement of anything that says what might be the weak link in the pressure containment boundry. Even test barrels had huge unknown errors for decades because the testers didn't understand what the CUP system was actually measuring. Even now the instrumentation has errors that aren't always accounted for.

    Sometimes it's the brass, sometimes it isn't. When it isn't the brass there are no pressure signs at all but it can still let go.

    The various action parts look undamaged right up until they let go on the next round. The stresses don't show, but they are there. Static pressures aren't necessarily the determinant - the dymamics of how stresses are applied can make a big difference - fatigue failure becomes a real possibility. Operating outside the design limits with no data is a prescription for disaster.

    There have been some spectacular examples of what can happen when that is done. The fact that one is operating outside the design envelope and nothing has broken so far is not an indication of any sort that the system has more capability than was deisgned into it, or that it won't break on the next round. The Challenger disaster comes to mind as a spectacular example but folks don't seem to learn from it. The principles are the same in reloading, just the scale is different.

    The difference between proof testing and normal design maximum pressures isn't margin met to be used for covering testosterone spills by those awash in the "right stuff" at the reloading bench - it's margin intended to cover the uncertainties and imponderables associated with pressure containment - material defects, tolerance variations, stress intensification blemishes like sharp scratches in a critical location, and so on. It isn't available to use for safely getting more velocity.

    There is a lot of merit to staying in the designed limits of the cartridge specifications. There is no rational basis for intentionally exceeding them in the name of a flatter trajectory or gaining another few feet per second. None.

    Fitch
     
  8. RockyMtnMT

    RockyMtnMT Official LRH Sponsor

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    I'll say it again, this is not some testosterone loaded, 'mine is bigger and badder than yours', how much farther can I push with a custom action than I can with a factory action question. I want to be safe when venturing into the unknown.

    Is quick load a good predictor for a wildcat cartridge that has no load data?

    Steve
     
  9. phorwath

    phorwath Well-Known Member

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    I can see pressure signs in my Lawton action with Lapua 300 Win Mag brass. OK, that's not 338 Lapua casings. And there are undoubtedly higher-priced and quality custom actions available. But Fiftydriver is always talking about reading pressure signs on Lapua brass with his 338 Lapua-based wildcats. Certainly some of those are custom actions.

    The primers still flatten. Any recesses in the bolt face (plunger holes or ejector slots) will begin to leave an impression on the case head. The primer pockets will loosen up in one or two firings if the Lapua brass is substantially overpressured. All of these case pressure signs will become manifest before the action lets loose.

    It may require a higher pressure load with a custom bolt action than a run-of-the-mill factory action before getting bolt lift resistance. Especially if the custom bolt face is without a plunger hole or ejector slot cut in the bolt face. But I suspect you'll still experience stout bolt lift before the custom action blows, unless you're unlucky enough to end up with an action with a structural defect or manufacturing flaw.

    No matter the design, make or brand of the action, I believe an observant reloader can ID excessive pressure by examining the fired brass casings. No different for a wildcat cartridge than the factory rounds. I've expanded 30-06 and 300 Win Mag Lapua brass primer pockets without working at it too hard.

    The most reliable method I know of to ID over-pressured loads is to reload the same individual casing over and over again during load work up. If the primer falls out of the fired casing on the 1st firing? The response is way to go! You're on the verge of a face full of torch-like, steel-cutting gas and metal. If the primer pocket loosens up within two shots, you're still heavily over-pressuring. Even if you get 4-5 firings before the case head won't hold a primer, you're still running them hot, but probably won't experience fireworks. If the primer pockets last 12 or more firings, you're probly operating within a safe pressure level.

    It generally doesn't take me very long to open up a primer pocket during my load development forays, and I still have my eyelids. :rolleyes:

    Now maybe someone with more seat-of-the-pants wildcatting experience than I have will sound in and share their experiences, and best advice, for identifying acceptable pressure & powder charges from the unacceptably high pressure & powder charges.
     
    Last edited: Mar 22, 2010
  10. Chas1

    Chas1 Well-Known Member

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    LouBoyd and Fitch, well said and great info for all to take to heart. Thanks for taking the time to share your knowledge.
     
  11. jmden

    jmden Well-Known Member

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    So far I haven't seen any of these well stated 'answers' answer the question at hand...

    edit: 'cept for phorwath who must've posted as I was writing...
     
    Last edited: Mar 22, 2010
  12. phorwath

    phorwath Well-Known Member

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    I will offer a differing opinion and state that the weak point is always the brass casing in any modern day firearm. The case head and primer pocket will swell under repetitive firings before a bolt, barrel, or action rupture, for the observant reloader who works up a load cautiously, while observing primer pocket and case head swelling. Without the expensive pressure testing equipment, I know of no better method of monitoring pressure. The brass will yield before the steel will. Unless one is hell-bent on destruction. Then they'll both yield at the same time.

    An accurate caliper can also be used to document case head expansion on the belt of belted cases, or just forward of the rim of the case head on non-belted cartridges. I use calipers when entering the world of the unknown in addition to monitoring primer seating resistance (or lack thereof).

    Now I'm not saying one can equate cartridge case head expansion to quantitative pressures, such as that last powder charge produced case head expansion equivalent to 68,981 psi. But if one proceeds with 1/2 grain powder increment increases for mid-capacity cases, and even 1 grain powder increment increases for large capacity cases, while working up to a reasonable maximum pressure load - I believe that dangerous over-pressuring signs can be read and identified on a fired casing before the firearm is damaged and lets loose. And by monitoring the case over repetitive firings, as described above, one should be able to ensure a selected powder charge is safe for repetitive use. Because the brass casing will always fail before the steel, when the operating pressures begin to exceed the brass case head strength.
     
    Last edited: Mar 22, 2010
  13. jmden

    jmden Well-Known Member

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    There wouldn't be less casehead expansion or primer pocket loosening with with a quality custom action vs. a factory action, would there? That area of the brass typically being unsupported, wouldn't it show pressure at the same pressures on a factory and on a more closely toleranced custom?
     
  14. RockyMtnMT

    RockyMtnMT Official LRH Sponsor

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    When I do load development with any of my wildcats based on the Lapua case, I do so using Norma brass. The reason being is that Norma brass is MUCH softer then the Lapua brass. In fact, I have found its very similiar to the same strength as remington RUM brass in that both will start to loosen primer pockets around the 65,000 psi level of chamber pressure.

    When using this brass in a rifle based on a Rem 700 and chambered in 7mm AM, you will see primer pockets start to loosen at around 3450 fps or a bit more with the 175 gr SMK. In a full custom receiver, say BAT, Nesika, or my soon to be released Raptor, primer pockets hold solid well over this. WHY?

    Brass fails, to some degree because it is not being supported well enough to prevent case stretching and case head expansion. What gives pressure signs is this case expansion. Thats why you get sticky bolt lift all things being equal in a rem 700 based rifle compared to a full custom that is much stronger. If the brass expands to much, you get sticky bolt lift and extraction, the more this case expansion is controled, that being by a stronger receiver, the less you will see pressure signs.

    Does this mean you can run the presssures through the roof, certainly not, that is why I use the Norma brass for a base line in reading pressures. I load it up to where you just start to see some change in primer pocket fit and then switch over to the Lapua case and consider that load max and you will never see any problems when using the Lapua case.

    The reason I do not recommend the Rem 700 anymore is not because it will not easily handle any sane pressures that the Lapua case should be loaded to, its simply because to the unexperienced or ego bound handloader, the Lapua case can support MUCH more pressure then any factory receiver should ever be loaded to. When loaded correctly, you will NEVER have a problem. I have loaded my lightweight 7mm AM very hot at times and I have used it for 7 hunting seasons now with no issues at all but I know its practical limits.

    Many out there may not have that ability to "stop" before getting pressure signs. In all honesty, if you are getting pressure signs with the Lapua case, your WAY over where you should be playing!!!
    __________________
    Kirby Allen(50)

    Allen Precision Shooting
    Home of the Allen Magnum, Allen Xpress and Allen Tactical Wildcats and the Painkiller Muzzle brakes.

    Farther, Faster and Flatter then ever before.

    kballen@3rivers.net

    This is what Kirby said about his action with the .338 Lap cases.

    Steve