The 9 round magazine that came with my Colt M4A3, as well as a cheapo 20 rounder I bought online both cause the same malfunction. About 70% of the time, the bolt holds open with one round left, and that last round has been popped out of the mag and is loose inside. I have a few awesome 30 rounders that I have had absolutley ZERO problems with, so i know the problem is with the magazines. weak spring? overpowered spring? bad feed lips?

now why does this malfunction never happen to any other round other than the last round? yes i know they are cheap mags, but can they be fixed?

Poorly made mags always have problems due to their poor tolerances.  The 9 rounder is a bastardized mag, so it can be considered one of the el cheapos too.  My advice, keep them around to practice malfunction drills or whatnot, but use the USGI 20s and 30s for serious use.  There are many available for sale for $12-$13 apiece nowadays.  

I wouldn't waste my time when I could buy new NY legal Colt Preban 30 round magazines for $17.00ea from eagle firearms.

Colt Preban 30 round magazines

What is stamped on the floorplate of the 9 rounder?
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Why two threads?
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