[Unclenick]

The Hodgdon data also uses Winchester cases and Winchester SR primers. If you use anything else you can get some serious differences. In early 2006 Charles Petty ran some .223 experiments with different primers lighting Reloader 10X under 55 gain Hornady V-max bullets. IIRC, he got 150 fps difference in MV for the same load using different primers, with Remington 7½ BR primers producing the most velocity and pressure and also the best 100 yard accuracy, but the worst velocity SD.

CCI altered their magnum primers to optimize them for spherical propellants in 1989. Some of the newer spherical propellants from Alliant and Ramshot are easier to light up and don't need special treatment, but when you're using the older sphericals like WC844 (H335), you'd do well to use those warmer primers. I've also had terrific consistency from the KVB556M small rifle magnum primers sold by TulAmmo and Wolf.

What SD you should expect depends on a number of factors, but if you seat your primers really well (err on the hard side) you can often get down to 10 fps, but there are a lot of factors involved. If the loads are accurate despite higher MV variation then you probably won't need to worry about it until you start to shoot beyond about 300 yards, where vertical stringing from the difference in velocity can start to tell.