There has been a lot of speculation in the difference in velocity between the .280 and 7mm RM on this thread. I already suggested 200 fps with the same bullet weights, which is about what the 4-to-1 formula comes up with, but I got curious--especially after somebody here suggested it was only 100 fps.
So I looked up actual pressure-tested data for the two rounds, on two websites that both use 24-inch barrels, Western Powders and Hodgdon. They also both test a wide variety of powders and bullets. Here are the top velocities for four bullet weights, 140, 150, 160 and 175:
.280 140--2943
7mm 140--3217
274 fps difference
.280 150--2825
7mm 150--3133
308 fps difference
.280 160--2746
7mm 160--3069
323 fps difference
.280 175--2676
7mm 175--2915
239 fps difference
The average difference across all bullet weights is 286 fps, both from 24" barrels.
Now, some people will no doubt point out that this isn't a fair comparison, because the SAAMI maximum average pressure for the .280 is only 60,000 PSI. But the max for the 7mm Remington is only 61,000, and all these tests followed the SAAMI guidelines--which in rifle pressures is practically zero difference. It's irrelevant whether YOUR handloads get a lot more, from either cartridge. Just about anything can be done by optimistic handloaders, who work up loads using "pressure signs."
Whether or not all this makes any difference in the so-called "real world" is another question.
