While blathering on my previous post, I completely lost track of my original thought (first thing to "go" is your memory, and danged if I can remember the second thing now...
). Anyway, here's what I recall of previous discussions on this subject:
1) Older "20-round" bushy mags with the blue follower snapped in over the green follower: Use as-is, but adjust feed lips so that the first round is held level when installed in weapon (most tend to hold tip "upward", causing feed problems) and smooth off the bottom edges of the feed lips...OR...remove blue follower, grind off the .556 "bump" on the green follower, machine/file a bit of a groove in the center of the now-flat follower, adjust feed lips for level positioning, and smooth off bottom edges of lips. These seem to have excessive number of unexpanded coils at the base of the spring, probably to limit capacity. About half of these can be removed,
if legal in your area, to add an extra round but still feed reliably.
2) Current "20-round" bushy mags with the black follower: Check for level positioning of first round when installed, adjust feed lips if necessary, and smooth off the bottom edges of lips. Tabs on the front and back (narrow) edges of mag are to limit capacity, but can be squashed back flat or otherwise removed to increase capacity,
if legal in your area.
3) Sig 556 "30-round" polymer mags work great right out of the box; hold 12 rounds of 450b.
...more or less...