I wonder if 25% more pressure changes the burning rate?
Do you find this to be an issue in the 25-35?
, I might have an example to answer your question, but it's only one example and it might not be representative.
The .25-35-117 and the .308 Win with 168 gn spitzers are ballistic analogs. Each has the same bullet SD, and the two are quite close in bore capacity. The latter is the ratio of net case capacity to bore cross section, and it can be expressed as inches of the barrel's bore; it is to case capacity as SD is to bullet mass. If two cartridges have the same SD and bore capacity, they will, at the same pressure, ideally use the same powders, with the charges proportional.
In Hodgdon's data, ball powders are clearly the winner for the .25-35. The single base stick powders manage about 2200 fps (or a bit over), but BL-C2 gets over 2300 and CFE-223 gets a stunning 2400. Raise the pressure to .308 levels, and the sticks make a come-back, roughly equaling BL-C2, and percentage-wise they close the gap with CFE-223. At starting load pressures, the .308 data starts to look more like the order of the .25-35.
I can't emphasize enough this is only one example. Hodgdon's .308 data appears to have been shot in different barrels over the years and was likely shot by different technicians. The bullets are of different construction, and different primers were used. For such a comparison as I've made, one needs to look at the averages over a wide range of such data. I do think, though, the data would show a "burning rate" chart of slightly different order for these two pressure levels. For that matter, if you change the cartridge proportions (SD and bore capacity), you'd get another slightly different chart.
tcp, I couldn't think of two cartridges in Hodgdon's data that could be used to compare a change on the order of an AI job. Well, there's the .300 H&H and .300 Wea, but that data looks strange. Instead, I compared the .308 and the .30-06 using the same bullet, and looked at the single base stick powders. Here, the net case is about 29% more (
well beyond the nominal 10% you mentioned), and I found the charge could increase about 14%. So, for each 10% of case, you get a bit less than 5% more charge. This is well below the 7.5% in the Lee "rules" and below the 6% suggested by my old copy of QuickLOAD. For two ball powders, I got more like 18% or about 6% per 10% of case increase --exactly the opposite of what I'd guessed, namely that single base stick powders would be closer to Lee's "rules" than the ball powders. Theory doesn't always match the real world! So, to answer your original question, each 10% of case will allow you about 5% more of the same powder, more or less. If you have some time and several good load books, you might compare the .300 H&H to the .300 Wea, as presented in one load book; none of my books have both in it.
