With the shorter bullets like the 300 gr Rem JHP, you don't need to remove the front rib, though I did bevel it ever so slightly to keep the case mouth from snagging on it as it's being stripped from the magazine. Hard to see in this pic but it's the best I can do with my limited photo skills:
With the longer bullets like the 405 Rem SP I had to remove the front rib and bevel it:
The only problem with that is it lets the front of the follower pop up when the mag is empty:
Though it will stay down by itself also - not really a big deal just something you should know:
The 325 Hornady FTX fits, barely:
But the Barnes 300 TSX loaded by SBR won't fit in a pmag, they're too long (Ok, one will fit but no more!):
If anyone else has a different experience by all means jump in
Edit: And I agree with Big Bore, I only modified one pmag and decided it wasn't really what i wanted - other mags are better suited.