Why does Lee state that you shouldn't use any Federal primers in their Auto-Prime? I had assumed that primers were quite similar in their construction and dimensions. This should mean that you can use any primer in the Auto-Prime. Is it really that dangerous to use Federal primers in this tool? What about other manufacturers tools?

I use federal primers in my lee hand primer with
no problems.

The design of the Lee Priming Tool makes a chainfire possible. There have been enough of them so Lee did tests on the various primers and concluded that the mass explosion of a tray full of primers would not hurt a person wearing eye protection badly IF the primers used were WW or CCI. The characteristics of the Federal primers are enough different so that a tray full going off all at once CAN hurt you badly.

I once had the primer feed on a Lee Pro 1000 (The basic design and meterials are the same.) chainfire and it made a believer of me. About 70 Federal small pistol primers blew and shot plastic everywhere. I don't even wanna think about 100 magnum large primers going at once.