I seem to notice in my handloads (sub “max velocity”) I get a slight cratering of the primer. Pockets tight and no bolt stiffness and primer not flattened. I don’t know what it is about the short mags, but it seems they are associated w more “cratered primer” than my other cartridges. Is this just me, imaging things or do others see it as well?

I have seen it even with factory loads. Combine 65,000 psi, softer primer cups, rough or oversized firing pin hole and you will have cratered primers. Most magnum primers have harder cups, I use CCI & Federal and don't get the cratering even with max loads.

I use CCI primers with my magnum loads.

I believe it primarily is about the fit of the firing pin to the firing pin hole in the bolt. If there is even a slight space between the pin & the hole this can cause cratering. Not to worry as long as primer pockets stay snug your not over pressure. My Weatherby Vangard 300 WSM does the same thing yet primer pockets stay sung over several firings.

Have your firing pin bushed and the cratering will go away. It’s from poor firing pin fit combined with the high pressure short mags are designed to work at.

I have a 300WSM.I noticed a lot of factory stuff at the range had some really cratered primers.Mine was one of those that could not be loaded to book max.In fact in my rifle if I used 180gr load data for 165gr bullets it was just about perfect.Mule Deer said the short mags really don't have a freebore.The rifling starts at the beginning of the barrel.That can really have a big affect on pressure spikes and seating depth can be very critical too.The WSM's are already loaded near max pressured around 63-65K,so small changes can have major affects on pressure.I fire lapped mine to give me a little freebore.It dropped my velocity on known loaded velocities of ammo I had about 100ft per sec.Think about that.It usually takes about 2 or 3grs of powder to get that extra 100ft per sec.I haven't been able to get back to the range to check my loads after I did the fire lapping,but I think I can be loading around the max book loads for the weight of the bullet I'm shooting now where I couldn't do that before.Here is what my throat looked like before and after shooting 20 fire-lapping bullets

Interesting. I did the fire lapping thing on an old rifle that hadn’t been fired much, I did 5,5,10,10 shots with increasingly fine grit. I noticed the distance to the lands increased 25 thousandths. That rifle doesn’t fowl hardly at all now and shoots just as good as it did before.

What does this mean? Not familiar with “have your firing pin bushed....”

Bushing the firing pin hole is where it is drilled out and a tube or bushing is threaded into the hole. The inner diameter of the bushing is then drilled for a precise match to your firing pin. This results in better support for the primer cup and no place for the cup material to flow into. Google it and I bet you can find photos or even tutorials.