Yeah, slamming isn't good for seating primers, which is why I don't use the priming "system" with Lee Loaders anymore, even when I use the loaders themselves. Had a Large Rifle primer go off when I was 13 when using the Lee Loader for my Mosin-Nagant, and never wanted it to happen again. And it hasn't!
The 'tap-in' method which is used by those little Lee Loader sets is the only "system" I have seen which involves that kind of sharp impact when seating primers. Fortunately it is also a very safe and contained method - probably the most contained since the primer is fully enclosed when it is being seated. And I have had 'several' primers go off when using Lee Loaders over the years.
AS far as I have been able to figure, you could get into trouble in two ways with them: once if you have unprotected eyes above the case mouth (where the primer explosion is released between the case neck and the primer seating rod...the tolerances being quite small there); the second could potentially happen, and this would be the bigger concern, if you had a quantity of primers exposed nearby which could be detonated by the ignition of the one inside the tool parts.
Open primers are certainly nothing to trifle with. 30 years ago my 'foolish curiosity' caused me to pursue a controlled experiment whereby I 'discovered' that primers in an otherwise empty case will propel themselves out of the primer pocket with sufficient force to penetrate cardboard boxes. Knowing that, I later disposed of perhaps 40 primers from old ammo by placing them in a soup can which was buried in soil with only the open end exposed skyward. When a crumpled paper 'wick' finally burned down and the primers went off, the sides of the can were perforated and the can bottom was well protruded.
While I don't load 'varmint ammo' by the thousands as some do, I have somewhat unconsciously made it a habit to load primers 20-30 at a time into primer trays anymore, even when I'm priming several hundred cases. Knowing what they can do.....
But back to the Lee Loader incidents, after a few 'surprises', I developed a method where I would hold the priming rod near the mouth of the case so I could gauge the seating of the primer with a few light taps and then stop. It seems that continued taps after the primer is fully seated is what causes Lee Loader 'surprises'.
