I recently read an article about that described measuring case head expansion (thick section of the case just in front of the base of the case) to be a reloader's way to "measure" chamber pressures. The article then proceeds to state that the principle behind this to be that if there is "X" amount of expansion it means that there is "Y" pounds per square inch.
The article further states that without a "standard" such as measuring a factory case of the same caliber before and after firing, CHE really cannot be determined. And another postulate tries to determine whether the CHE should be conducted on once-fired cases or new cases.
I find CHE interesting because I have a wildcat that I am trying to develop loads for, .270 Ackley Improved, and cannot even find load data (minimal at best) for this cartridge and would like some form of standard while finding a load for the cartridge without going over safe limits.
What are other's thoughts, ideas and/or experiences with this process/concept?
I have been doing this for years, despite the fact that most guys poo-poo this method. One of the things that puts guys off is the fact that you can only make the correct measurements if you use a blade micrometer, since the diameter of the cartridge rim is greater than the diameter where you want to measure on the case head. The walls of the cartridge will also be of a slightly larger diameter after firing, just ahead of the web where we want to make the measurement, so a regular micrometer will be measuring the larger diameters, rather than the diameter of the case head itself.
I C-clamp my blade micrometer to a flat steel plate, and stand the cartridge case on a flat steel washer that elevates it just enough above the steel plate so that the blades of the micrometer are getting down into the area just above the extractor groove for an accurate measurement. This is the only way I have found to make the measurement at the same height every time. It has to be repeatable, or the measurement will be worthless to you.
I also make a mark on the side of the case to identify the spot where I made the measurement, so that I can measure at exactly the same spot after it has been shot. Cartridge cases aren't often perfectly round, so this is necessary to do. I measure all the cases in my test series, and mark the diameters on the side of the case. It is necessary to measure to .0001", since we're looking for that amount of expansion. It is also necessary to use new brass, which has not yet been work-hardened, or our measurements will be worthless. I check for expansion right after firing, and write the number on the case, right next tot the unfired diameter number.
What I'm looking for is expansion at the solid web of no more than .0005". That is my absolute maximum, and for loads to be used in field conditions, I go no higher than.0003" of expansion. I have found that almost all the factory loads I have shot expand around .0003", so I know that I won't have an over-pressure problem at this level.
There is a fair amount of data out there about this, and I have a reloading manual written by a guy from Australia named Nick Harvey that has an entire chapter on this. What he does is to file down the rim of the cartridge case, which enables him to use a regular micrometer to take his measurements right ahead of the extractor groove, without any interference from the rim. I tried this, and it was rather tedious, but it would save a guy a couple hundred dollars on buying a new tool. It also ruins the cartridge case for further use, but sacrificing a half dozen or so cases to save the cost of a blade micrometer might be OK with you. If you're going to try several different powders & bullets, you might just want to buy the correct tool and save the cases. In any case, the title of the book is NICK HARVEY'S PRACTICAL RELOADING MANUAL, and it is a very good read. My copy is dated 1993, and I don't know if it's still in print.
Another guy who has worked with this method is Dave Scovill, who writes in Rifle Magazine and Handloader Magazine. He did an article many years ago about this, and had his loads pressure tested in some bullet company's lab. He listed the measured pressures next to his case head expansion figures. In most cases, .0005" of case head expansion coincided with 60,000 psi. ( This is why I use .0005" as my absolute maximum.) I can't remember what the old Hornady manual said about this, but it seems to me that this number was in the ball park, as it is in the Hodgdon manual.
There's another way of doing this, and Ken Waters was the guy who wrote it up many times. He measures at the point where the case walls expand, just ahead of the web. ( He calls this the "pressure ring.") This is a entirely different measurement, and he found it to be reliable for determining maximum safe loads. I have tried it, but prefer to measure on the web rather than ahead of the web, since the chamber diameter is a player in how much the walls will expand. Measured back by the extractor groove on the web seems to be a more like the concept of measuring a copper crusher, like they used to do in ballistics labs years ago.
Lastly, I'd like to say this : It's important to remember here that what we're talking about is not a measurement of pressure - it's a measure of
relative pressure. I've been using it to approximate factory load pressures, and for that it has been pretty reliable. If you want to go beyond that pressure level, this may or may not be useful to you. With Ackley Improved cartridge designs, the straighter case walls are said to grip the chamber walls better than the tapered un-improved cases do, possibly decreasing bolt thrust. This would mean that using this method may be less useful with those designs. I have used it with the 30-06 Ackley, and found that it worked just fine for me. The only problem is that there is no factory loaded cartridges to use as a control, but loading to .0003" case head expansion with a new ( un-fireformed ) 30-06 case posed no problems. My velocities were right where I had thought they would be, and I used the loads extensively for many years with no problems. Hope this is all helpful for you - best of luck to you in your endeavors.
