CAUTION: The following post includes OPINIONS beyond or not covered by currently published maximums for this cartridge. USE AT YOUR OWN RISK. Neither the writer, The Firing Line, nor the staff of TFL assume any liability for any damage or injury resulting from use of this information.
I have a love/hate relationship with pressure questions. Hate, mostly because if I give my honest opinion about some things, some people might think I am advocating something unsafe. I am not. DO NOT do anything you feel is unsafe, and don't take my word that something is safe for you to do, just because I said it had never been unsafe for me.
First, there is our current, multiple means of measuring pressure. Different measuring systems The same amount of pressure is a different number using c.u.p., psi, kg/.cm2 or stone per square rod.
So, to be an accurate comparison, all the numbers must be from the same measuring system.
Second, Industry standard maximums for cartridge pressure are NOT maximum
possible pressure limits they are well below that. They are safe
working limits. They are what the industry decides is a good place to stop, in order to be safe in the majority of arms in each particular caliber.
They are intended to be the max that the "usual" guns in each caliber can handle with MINIMAL risk.
Proof loads are well above working ammo pressure standards. They are a significant overload, compared to standard ammo. And each gun proof fired survives the proof load, without damage. This should illustrate clearly that exceeding industry working maximums
slightly does not instantly, irrevocably and unfailingly turn your gun into a grenade.
Modern bolt action designs are pretty strong. Some decades ago, I was told that Remington proofs their 700 series actions at 80-90,000psi. I cannot verify this info, but it seems reasonable to me. I would expect Savage to do something similar.
Think about that for a moment. Depending on the exact cartridge chosen that about a 1/3 (or more) overload pressure, compared to regular ammo.
The rifles survive, unharmed, and then we buy them!
What chambering the rifle is in (mostly) only matters when it comes to what it takes to generate the pressure in the case, not how the rifle handles it.
Very large and very small rounds that significantly change the amount of metal around the chamber of a given design could make a difference in ultimate blow up failure point, but we're not talking about going anywhere near that, right??
At a certain pressure level, even the best supported brass begins to fail. I have heard that depending on specifics (including rifle design) this can begin as low as 65k psi. (the beginning of the failure cycle is the classic 0.001" expansion of the solid head of the case).
Reading Uncle Nick's post I learned that some folks consider half that to be the limit. I had been unaware of that, previously, thank you Uncle Nick.
I'll take a shot at some of your questions, pull the grandpa thing and tell you a story from the ancient past...
Does the brass have enough of a factor that it become the driving limitation?
The brass is THE limiting factor. Meaning, it is going to be the first thing to fail. And it should be! How well it is supported, quality of the brass, and thickness at vital points determine where on the pressure scale it will fail, but it will fail before the steel does, or something is drastically wrong.
the question I have are "what are the limitations" for a Modern Bolt Action rifle using the 7.5 Chambering?
I would say at least 80k psi for a modern action, probably much higher (just a guess, I have no data other than 40+ year old hearsay) The case, somewhat less but how much less involves a lot of factors.
Can the modern load programs give you an idea on what the Pressure that combination can take.
Computer programs still work on GIGO. Garbage in, garbage out. Meaning accuracy of result depends on accuracy of input. Very good at doing arithmetic calculating pressure generated using default values and variables you give them. Can't calculate what that pressure will do in YOUR rifle, with all the additional variables involved.
In other words, they can calculate load X generates 73,200psi. Can't say what 73,200psi will do in your gun, with your brass. (and yes, I deliberately picked a high number for dramatic effect.
)
While I would like to explore the upper but safe limits I would like to get the input on it and if its negative, then not do so.
"Safe" is a term with widely varied and very situational meanings.
IF you go beyond recognized industry standards you go beyond what any 3rd party will recognize as safe and prudent, and whatever happens is entirely your, and only your responsibility. Such is our world, today.
Now, my personal pressure horror story..
having all the wisdom that comes with being 16 and reloading for about a year, I was confident that I would not make a mistake. And I didn't. I made several.
Rifle :Remington 600 .308 Win
intent: "plinking" load using 173gr cast bullet and bullseye powder.
Mistake #1: Using an improvised tool for the job.
I didn't have a powder trickler, so I used a fired case, instead.
Mistake #2: (mistakenly) using a sized, primed case, (ONE TIME!!!) instead of a fired case as the powder trickler
Mistake #3: Putting that case, about half full of pistol powder into the loading block. Mistake #4: Seating a bullet in that case. Mistakes #5 & 6, not realizing what I had done, and firing the round.
Got hit on the cheek with gas. (ALWAYS WEAR SHOOTING GLASSES!!!)
Bolt frozen shut. Fortune favoring the fool slightly, the overload was the last round in the magazine.
Off to the gunsmith. Old family friend, distant cousin, and a very good smith. I got quite the talking to with the final damage report.
Broken extractor. Broken Extractor pin. Broken safety pin, broken sear pin. Said he did everything short of x-ray and couldn't find any cracks in the bolt or the receiver.
I don't have the measurements of the fired case anymore, but I have kept it as a reminder of stupidity. Case head expanded enough that the primer fell out and cracks in the web could be seen. A "belt" of brass was swaged around the body of the case, at the front edge of the solid head.
According to the best info I could find at the time it took between 90-110,000psi to do that. The rifle survived, slightly damaged, but repairable, and so did I.
Hope this long rant gives you some things to think about.
