Originally Posted by RockyMtnMT
If the pressure does not show the same in a custom action, how do you know when you are there?
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.