I am by no means an internal ballistics expert by any means but I have read many studies and tests about the topic and can tell you a couple things.
First off, the smaller the head diameter of a round, the higher chamber pressure it will handle with a given brass quality.
By that I mean that if you take three cases, a 223 Rem, a 30-06 and a 300 RUM, all made with the same alloy brass, with the same case head hardness and the same case wall thicknesses, in most cases, the 223 will handle more pressure before showing pressure signs then the other two larger cases.
From what I understand, its a simple function of square inches and cubic inches in relationship with the PSI.
Here is another example, take a round, any round, does not matter which and shoot it in a rifle with a firing pin and hole in the 0.060" range. YOu will easily be able to take that cartridge and load it up to VERY high pressures possibly even +70,000 psi before you see any primer cratering around the firing pin.
Take the same rifle, enlarge the firing pin hole and firing pin diameter to say 0.070" and you will see primer cratering at much lower pressures, from what I have seen and read, I would say around 55 to 60,000 psi you will see primer cratering around the firing pin.
Take that rifle and enlarge the firing pin to 0.075" to 0.080" which is not to uncommon with some factory rifles such as Win M70s and you will get primer cratering at any pressure.
So why the difference, its simply because the area over the firing pin is basically unsupported and as that area increases, it gets more difficult for the primer cup to withstand the pressure because there is more area that is unsupported.
It is similiar with the cartridge case itself. In a 223, it can be nearly impossible to read pressure signs until its really to late.
As the case diameter increases it becomes easier to read the pressure because the larger head diameter will create more bolt thrust at lower pressures and you will start seeing pressure signs such as extractor marks, cratered primers and heavier bolt lift.
Comparing a 223 with a 30-06 is nto a good comparision, comparing a 30-06 to a 300 RUM is an even worse comparision, apples and oranges. I can assure you that there is no RUM cases out there today that will handle 70K psi and keep a tight primer pocket.
The reason I say this is because I have read pressure data on this round and from all the ballistic data I have read, if your pushing close to or over 3400 fps in a 26-27" barrel with a 180 gr conventional bullet, you are pushing very close to the 70k psi range and your primer pockets will loosen.
Another thing to keep in mind is a theory known as "detonation". I have read and heard lectures about this condition and basically what the thinking behind this is is that when the internal pressure reaches a certain point, generally thought to be between 72 and 75K psi, whatever remaining powder that is in the case/barrel will burn almost instantly which will result in a severe pressure spike.
This is a relatively new idea that has been being studied and the reason is because it has only been the last 10-15 years that we have had the correct powders to see this problem.
In most cases, there is so little powder left in the barrel when this pressure is reached that you see very little if any pressure spike. With rounds such as the 06 based and even most belted magnums, there just is not enough powder remaining to see a real issue when this is thought to occur.
Now, with the rounds such as the RUMs and the big Wby and some of the big Wildcats we are playing with, it is thought to happen much more often. The reason, is because we have powders such as H-50BMG, Vhita Vouri 20N29, WC860 and WC872 that we can load in very large capacity rounds.
In past days you may have only a couple gains of unburnt powder when the internal pressures reach this point of Detonation. Now, you may have many times that amount and when it happen you see the primers being blown out of the cases, not loosened, blown out!! These are the ones that fall out of the case when you eject a case.
Many top internal ballistic experts believe in this theory, some do not but it does make alot of sense. Anyone that has pushed a large capacity round to its limits have seen instances where some cases do not show any real excessive pressure while others have leaky primer pockets. This is what is believed to be the reason for that.
Back to the 300 RUM. I will admit that the Federal 300 RUM cases are harder in the case head then the Rem cases and would likely take more pressure but I have tested both of these brands of brass in several 300 RUMS with barrels in the 26 to 28" range and all have topped out in the 3350 to 3400 fps range with absolute max loads, that being where the primer pockets begin to loosen with a single firing.
Loading the 300 RUM to the same head expansion seen on factory ammo will get you around 3250 fps with a 180 gr pill. In pressure barrels, the factory testing has proven that the 300 RUM will push slightly over 3300 fps at remain in the 65,000 psi pressure limits at normal shooting conditions.
Again, not to be disrespectful of Mr. Watsons tests with the 30-06 but I am sure if you asked him he would tell you the same thing, that a comparision between a 300 RUM and a 30-06 would be nearly meaningless as far as top end pressure limits.
If you understand and believe the theory of Detonation, his claims that he was getting near 80,000 psi would raise some questions with no pressure signs but I have never read a test of any kind using conventionally designed rifles that would allow the use of 80,000 psi with no pressure signs on the case.
Its funny you bring up "fancy" brass from Norma and Lapua. First off, if you want soft brass that will let loose early with pressure, get some Norma cases. I believe there is a reason Wby uses norma cases. Yes they are very good quality cases but I have on many cases seen once fired factory Wby cases that had primer pockets nearly to loose to reload!!! This was in a 300 Wby. We shot those loads over a chrono and I believe that they clocked just over 3200 fps with a 180 gr bullet. 3230 fps if I recall correctly.
We took some Rem cases and matched that easily and were able to get 3-4 good firings per case with matching performance levels whereas the Norma cases were done after two firings.
Lapua cases, yep, they are stout. If there is a case out there that may take 80,000 psi my guess would be the 338 Lapua case from Lapua. I have used this case on many of my personal wildcats and driven it HARD. I have yet to loosen a primer pocket on a 338 Lapua case and again, I have pushed them as hard as I can with apporpriate loads. Do I think they would take 80K in pressure. I do not believe I have ever pushed them that hard and do not wish to because there are other issues involved with that level of pressure. No need to play with that snake, eventually you WILL get bit.
I am sure you did not mean any aggression by calling me THE EXPERT HERE but I will be honest in that I do not like that label. There are dozens on here that know far more then I do about a good many things. I am no different then you or anyone else, my full time job just happens to be making rifles.
I try not to come off as a know it all because I certainly am not one. If I have come off that way to you, I apologize.
Good Shooting!!
Kirby Allen(50)
