Tex,

I'm glad you clarified the "overpressure event" in your follow up. What concerned me the most in your original post was your assertion (an incorrect one) that your listed load was no where near a max load. I don't know where that info came from, but if you're extrapolating data from other sources, this is quite a bit off from the Hornady manual. 43.8 grs is listed as a max load, and you're 0.3 grs under that, and thinking that you're at a low to mid scale load (your follow up post). This is a huge red flag that something is quite seriously wrong, and a FULL STOP is in order.

As far as velocities go, there are many possibilities why your velocity is "off" from the book. Firstly, the Hornady manual lists that max load as having a 3100 fps muzzle velocity. These test loads in the manual were fired from a Winchester M70 with a 24" bbl. First, there are differences in rifles, chambers, barrel lengths, and even how "fast" a barrel is. Second, there are differences in measuring equipment, aka chronographs. If you took 3 or 4 different rifles, all chambered in the same cartridge, with the same barrel lengths, you would see differences in muzzle velocity when shooting the exact same load over the same test chronograph. Similarly, if you took one rifle, with identical loads, and fired them over multiple chronographs, brands, styles, etc, you will also see a difference in measured muzzle velocity. I have seen rifles that were a couple hundred fps off from each other when shooting the same load. I have also seen chronographs that were several hundred fps different from each other when clocking the exact same rifle and load. That is just The Way of Things.

Darkker,

Reading into my words that the use of magnum primers will bring the end of the world is a huge disservice. That is not what I said, nor what I meant. You are correct that each rifle is it's own individual animal, and needs to be treated as such. Just because you (or I) have a rifle that switching to magnum primers in makes "little difference" is irrelevant. Some barrels, chambers, throats will take the switch with no problem, and some can be a big problem. But for a loader (any loader) to take a max load, with the bullet jammed into the lands, and proceed to use a magnum primer where the load recipe called for a standard primer, should only be done with caution, and only done while slowly working the load up from scratch. Doing so invites an initial pressure spike, as the increased ignition wave due to the bullet not moving sooner in the wave cycle. It can be done, but must be done carefully, and with purpose. Not just swapped in to see what happens (not saying that is the case with the OP). But switching to a magnum primer in a max load, where the bullet is already jammed into the rifling gives the chamber (in this case actually it's the cartridge case that's the "chamber") no where to go. The bullet will sit motionless for a longer period of time, until pressure builds up behind it enough to force it to move down the rifling. Where as if you are jumping bullets, the bullet starts moving almost immediately, as soon as pressure builds enough to overcome the neck tension on the bullet. So the initial ramp up in the pressure wave changes when you jam bullets. The initial pressure wave also changes when you use a magnum primer. More of the powder charge is ignited in the initial instant of ignition, so the pressure wave starts off sharper. When you stack those two occurrences, jammed bullets, and magnum primers, you can have an initial pressure spike in some barrels/chambers that you will not see in others.

So when a loader/shooter is having issues with pressure coming on "early", or velocity that seems too high, AND he is jamming bullets with magnum primers, then caution and load reevaluation is in order. But when a loader/shooter states he is shooting a load that is no where near max, but in fact, he is right at book maximum for that load, then EXTREME caution is in order, and a full stop is necessary to evaluate what the heck is going on.