Different testing equipment for one. They used a tighter chambered, shorter throated, 26" bolt action rifle. That said, Hornady's data is conservative to put it mildly. In a 20" Colt 1/7 twist barrel AR 23.2 grains of H335 got 2846-2863 fps. That was using H335, RP 7 1/2 primers, RP and WCC cases, Hornady 55fmj's seated to 2.218-2.220".
Hodgdon's data in my tests with my rifles shows that their data all the way to maximum showed no pressure signs with my components. The result of shooting the maximum of 25.3 grains of H335 with the above components got 3162-3200 fps. Start lower and work up using Hodgdon's data for the AR-15 and if you load for a bolt action especially.
Using Hornady's 55gr Spire Point bullet, 23.2 grains of H335 got 2829-2845 fps using the above cases and primers. 24 grains using H335 and either 55 fmj's or 55 SP bullets got 2960-2975 fps in my AR. Outdoor temperature at the time of my test was 90 degrees in July. In my AR's I've settled on 25 grains of H335 and it shoots nicely with no pressure signs in different .223 cases. These results are from only one testing session using an Oehler 35P chrono.
