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Custom action pressure ceiling.

 
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  #1  
Old 03-22-2010, 09:21 AM
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Custom action pressure ceiling.

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
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  #2  
Old 03-22-2010, 12:55 PM
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Re: Custom action pressure ceiling.

Quote:
Originally Posted by RockyMtnMT View Post
If the pressure does not show the same in a custom action, how do you know when you are there?
Steve
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.
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  #3  
Old 03-22-2010, 01:28 PM
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Re: Custom action pressure ceiling.

Quote:
Originally Posted by LouBoyd View Post
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.

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.
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  #4  
Old 03-22-2010, 04:54 PM
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Re: Custom action pressure ceiling.

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?
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  #5  
Old 03-22-2010, 05:39 PM
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Re: Custom action pressure ceiling.

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.
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  #6  
Old 03-22-2010, 07:37 PM
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Re: Custom action pressure ceiling.

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
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  #7  
Old 03-22-2010, 07:54 PM
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Re: Custom action pressure ceiling.

Quote:
Originally Posted by Mikecr View Post
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.
+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
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