Re: best place to measure web expansion
When I was working up a load for my 30-06 with H414 I carefully measured CHE to within 0.0001. I increased powder until I got a readings. I found that if I let a round sit in the gun that was hot that the reading was higher. ie; #5 of a string. This I felt would be the worst case scenario. H414 is a temperature sensitive powder and on a hot day in a hot gun pressure can soar. A load that would be 0.0002 cool would give 0.0010 when hot. After several loading's cases failed (loose primer pockets). My personal take on it is this, if a hot round in a hot gun produced 0.0002 to 0.0005 thou expansion it was a hot load. I backed down at that point, I then checked a load with one less grain of powder, it gave no expansion (hot and hot test). This was my max load, a do not exceed. I have to agree with Speer on this, if there is any expansion on the case head it means that the yield strength of a new case has been exceeded. Back 30 - 40 years ago some factory ammo was loaded hot, brass was not reloadable, primer pockets loose on the first firing.
Remember, a fired case that does not show any CHE may still be unsafe to fire, it may exceed the design pressure of the rifle. 0.0002 thou CPE in a 30-30 would be super hot for an old '94, perhaps OK in a Ruger #1.
Of course all of this measurement is on a new case, not one that has been previously fired. Brass work hardens, becomes stronger with each firing, and a little more brittle also.
I now measure PRE for load intensity comparison and I read the pressure ring position. This is especially easy to read on magnums. For belted mags I just look at the shinny ring between the headspace or belt edge and the first contact that the case has with the chamber, anything less than 0.025 is what I consider hot, anything more that 0.050 is normal. What I like about PRE is that I can measure it on a reloaded full length sized case as well as on a new one. However, it will not tell you if you have exceeded the cases yield point. The rifle chamber contains the pressure of the sides of the case, the brass head contains the pressure at the rear, the part that points at your face. The head of a brass case will literally disappear if overloaded enough. Brass will yield completely and flow as if molten, blowing the crap out the rifle it is in.
The problem with all these measurements is that the hot indication is not hot enough for most people, the desire is there to make ammo that exceeds all factory ammo and the book data loads as well. Factory ammo can be loaded hot, they expect the cases to be thrown away after firing.
Today, it very rare for any of my loads to meet or exceed any published data maximum. If I need more velocity, I need a different caliber. If the combo is not accurate, I need a different combo. May not be what I want to hear but it is the truth.
If you are reloading a common caliber all this measurement stuff is somewhat moot, lots of published ammo, lots of factory loads. This is handy for variants (like a custom short throated 300 Weatherby), special bullets, wildcats, or if you just bought 100 lbs. of non canister powder cheap. I once bought a bunch of powder called #44, it was cheap, had a wicked muzzle flash but it was cheap and it was fairly accurate. Did I mention that it was cheap. All it came with was a piece of paper that said any listed load for 4895 should be safe.
The powder companies aren't stupid either, they slowly change the powder, making it safer. for example if 50 gr was a max load in 1990, a new lot today may need 50.2 gr to be max. They sure as hell won't put out a batch that only needed 49.8 for max. If I have an old lot of powder I'll consult old data before loading to what is in a new book.
It's nice to know the origin and some history of the CPE/PRE argument.
