Can I 'download' factory 12ga. buck (reduce)

I have an old Geha? Mauser 12ga. bolt shotgun, good and solid but well worn stock refinished (real kid butcher job) that I have a bunch of fun with in a large country junkyard (all kinds of targets). I recently bought a box of Estate # Buckshot loads. They were on sale and cheap and didn't say anything about Hi-Vel or Magnum so I didn't think they'd be too bad. Wow, do these things kick in this gun, downright unpleasant! Found muz vel listed at 1325, is that hot? Now here's the groaner: If I open these things up and remove maybe 20% of the powder charge can I 'tame' these down for easier recreational shooting. I used to empty out shells when I was a kid and fill'em with confetti or pop corn just to tick off the old man. It worked, but I still have bumps on the butt. Funny tho!