Tallest,
I believe you are in error. I'm pretty sure Hodgdon negotiated with Hornady to get this powder on the market and to be able to use the Superfomance trade name, and it is why Hornady's name appears on the label. Same with Leverevoltion powder. But there is a difference in the way commercial ammunition makers develop loads and the way load manuals recommend loading powder. The SAAMI standard calls for the Maximum Average Pressure measured in testing to be the not-to-exceed pressure for an average of 10 rounds, with shot-to-shot pressure variation putting some of the 10 rounds above that number and some below it in a maximum pressure load (commercial makers don't always load to maximum pressure; they often stop wherever a particular lot of powder happens to hit their velocity target). A separate number called Maximum Extreme Variation limits how wide the spread from lowest pressure round to highest pressure round in the ten can be. They can do this because they have pressure test guns instead of published load data and therefore don't need to worry about powder lot burn rate variation.
The load manual authors do not do that. Because they have customers who rely on load data that doesn't change with powder lot burn rates, they have to use a more conservative number to avoid having customers exceed the published data when they package a faster lot of powder. This is why the maximum loads you see listed by Hodgdon, for example, have pressures that are below the limit for the cartridge. They are not allowing the highest pressure round in the 10 round sample to exceed it. The plus to this is, you can look at their data and know the powder whose maximum load is highest is the one which had the lowest pressure variation with the bullet listed in their test. But it also means their maximum pressures are lower than the maximum an ammunition maker may reach by being able to measure his particular powder lot's performance in a pressure gun. This is one of the reasons for handloads not achieving the same velocity the commercial ammunition does, even if it's loaded with the same powder and bullet.
Yosemite Steve said:
If I show no pressure signs I will also try three loads with mag primers because this is a slower burning powder and my rifle is only 22".
Barrel length doesn't dictate a change in powder burn rate until the barrel is very short. If you had a single-shot pistol or a licensed SBR in your rifle chambering that had an 8" barrel or shorter, then you might find a faster powder could produce higher velocity at the same peak pressure as a slower powder was reaching. But for normal rifle barrel lengths of 16" and up, the powder that produces the highest velocity in a long barrel will also produce the fastest velocity in a shorter barrel.
Magnum primers sometimes produce more velocity but occasionally produce lower velocity for a couple of reasons I won't get into here. What you really want to do is take a chronograph and look at velocity standard deviation. If the magnum primer makes it lower, use that. If it makes it worse, don't use it. Because magnum primers can produce higher pressure equal to around 5% increase in powder charge, always knock the powder charge down 5% when you change primers and work back up. But you may see no difference. The main thing is to use the primer that produces the lowest MV SD, and then adjust pressure and velocity with the powder charge rather than with the primer. The powder throw control is more consistent.
