The vast majority of stoppages I have personally experienced with the M-16 and M-4 have been due to magazines.Issue magazines tend to dent easily, the "lips" get bent, and as they are frequently left loaded for long periods of time, the springs wear out. The Army improved them once with the new green follower, but it still binds on occasion. Plus a lot of magazines soldiers are using are old, beat and generally neglected. We were issued new HK 30 round magazines when we deployed to Iraq, which are excellent. They are much more heavy duty than the issue mags and the follower moves easily without binding. I also have a number of the plastic Magpul magazines which are just as good (IMHO) and have the benefit of being lighter (the one possible drawback is that they are a bit thicker, and as a result fit more tightly in a magazine pouch - but for me, thats a plus - they don't fall out). Don't let the word "plastic" fool you - they are made of some hi-tech material that withstand more abuse than even the heavier steel HK mags.
So a couple of quick fixes to M-4/M-16 reliability issues (other than keep the damn thing clean):
-Get new issue mags and take care of them (to include cleaning them - a lot of folk never clean thier mags).
-If you identify a bad mag - crush it. It keeps the supply guy from issuing it back out again to someone else (which I've seen happen).
-If possible obtain quality aftermarket mags - Magpul & H&K are the best I know of.
-Don't store ammo in mags for extended periods. If you have enough extra mags, rotate them periodically. Magpul mags have a neat cover that you can clip on the top that somehow prevents problems when leaving ammo in them for extended periods.
-Only put 28 rounds in each mag. Stuff 30 in and you increase the likelyhood of a failure to feed stoppage.
Adrian