My Winchester- and PMP .308 win cases differ by 17 grain (PMP are heavier thus less volume). Is there any way to calculate the different weight in powder you have to use to get the same pressure or speed if you are using the same powder and bullet but different cases? Somebody at our range suggested 0.1 grain difference in powder for every 4 grain difference in case weight. Any comments?

I think that depends on several things:

1. The particular powder.

2. The particular area of the loading curve you're working with. I've found that in certain areas of load development with a particular powder, there seems to be a linear increase in velocity and/or pressure signs. When one gets toward the maximum load, a little change in the load can make a relatively big difference in pressure signs.

3. The cartridge size is another factor. A 4 grain difference in case size in a 375 H&H Mag. case is a whole lot different than a 4 grain difference in a 222 Remington. You also can't really say that the increased weight is because of decreased case volume. For instance, trimming the neck of a case will bring down the weight with no change in case volume.

In my opinion, you would have to evaluate each particular cartridge/bullet/powder combination to determine what powder adjustments were needed for a change in case volume (provided there really was a difference in case volume). I know from experience that my adjustment for 308 Win. loads with Federal brass vs military brass is probably going to be 1 to 1½ grains less of IMR 4064 in the military cases. I found that out by personally testing the respective cases in my rifle (your results might be different). I have never bothered comparing weights of cases.

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