The main concept behind the "improved" cases is to create additional powder capacity so that you can drop down in burning rate and get enough additional powder in the case to affect an increase in velocity made possible by the slower burning rate.

The slower the powder, generally, the more volume of it is required to get to max pressure.

Just for purposes of comparison, a max pressure load of 4895 will only produce so much velocity with a given bullet weight in the standard case. If you go to a slower powder to extend the pressure curve over a longer time period down the barrel, you run out of case capacity before you get an increase in velocity. You probably cannot get enough 4350 in the standard case to get up to max pressure, but with the increased capacity of the improved case you might.

In the real world, it may be 4064 or 4320 for example that will give the increase rather than 4350, but I think you see the point regarding needing more capacity for enough slower powder to make the velocity increase.

The effects are really dramatic with smaller, highly tapered cases like .250 Savage and .257 Roberts Improved, but with a 7mm-08 Improved, you are not increasing the shoulder diameter.... only changing the shoulder angle ala "opening an umbrella" bringing the shoulder up and thus creating more capacity under it.

Ref. 7mm IHMSA, this is the smaller .300 Savage case which has less capacity than even the standard 7mm-08.

For most of the "improved" cases, the general rule of thumb is to start with the slowest powders used in the standard case.... loads that are usually compressed.... and work up from those max loads. Always start below book max. of course, but work up to max pressure like you would for any other case.

If you still have room in the case, then drop down to the next slower powder and work up to max again.

For example, with a given case you max out with 4350, but still have some rattle room in the case, go to the next slower powder like IMR 4831.

OR, if you max out with IMR 4831 and still have some room left, then go to H-4831 which is a tad slower.

Everyone wants to lean on "book" loads, which I feel very strongly is a big mistake. I get cold chills every time I get the feeling that someone is grabbing a load from a book without any "feel" for how to determine max pressure. Reloading manuals are NOT cookbooks, and to just grab a load, say a "Hail Mary," dump it in a case, then pull the trigger on it is asking for trouble.

I have and have used all but about a dozen of the 500 plus chamber reamers on hand and have never blown up a barrel or gun yet. I guarantee you I DO NOT have loading data for the vast majority of these and test fire every one of them myself. Not to brag, but the point is that you should use reason and discretion over leaning on a book..... especially when you get into custom chambers where dimensions are often tighter than the SAAMI type chambers the books are based on.

What do I know about "improved" cartridges? Just had P.O. Ackley, the man himself, as a personal mentor for 10 years. That's all.


Mike Bellm