I don't want to move the thread in side ways direction but will make a few addition comments here on loading large cross sectional cases.

While true double charge are a problem these are and should be be readily visible and apparent to the hand loader. The problem is that with dense fast powder the pressure rise very fast with just small change even in these large volume case (some refer to this as being spikey).

In the 500 S&W case you cannot see a 1 grain difference in the charge of a powder like Tite group.

The point here is don't need a double charge of fast power to cause a problem a feww grains will get you there and even the sharpest eyes won't detect the problem.

apologizes to Roscoepc for getting a little of topic in his thread

be safe and choose your powder wisely
Ruggy