Just my experience with a Bambu P1S using Sunlu PLA + 2.0 and Siraya Tech Pet-CF, every SL-15 lower I've printed (qty 6 or so) was too small for pmags and slightly tight for metal mags. None of the lowers were able to free drop a metal magazine. Minor filing was required for metal mags to drop free, but quite a bit of filing was required for Pmags to drop free.
I would insert and remove the magazine a bunch of times until i could see wear on the ridges in the magwell and then I'd file those points down. On one of my PET-CF lowers I had to file the sides to get the Pmag to drop.