Velocity difs alone are routinely above 25 fps in the same box of factory loaded rounds and pressure difs are routinely well above that level of difference. This has been written up and reported by professional ballisticians time & again.
Re pressure testing, I've had Tom Armbrust--the same chap who worked with Sherman Bell on his "Finding Out For Myself" series in DGJ--pressure test several reloads for me, and a couple of factory loads. On the factory loads (Eley Blue 12ga, Gamebore Traditional Game 16ga), they were very consistent: 20 fps max spread between high and low on the Eleys, 3 fps on the Gamebores. Pressure varied 200 psi high to low on the Eleys, 400 on the Gamebores. Some of my reloads did as well as the Eleys in terms of velocity spread, but a couple were in the 50+ fps range, max spread. Pressure variations on those with the 50+ fps spread were as great as 1,000 psi--which is why it's good to build in a "cushion" when you're working for low pressure reloads.
