I found my M4s to be far more dependable than any M16 I ever used. M16s were garbage, and they are still in use by Fobbits.

The only time I ever had any issues with an M4 was when it was filthy on the inside. The main problem with that is that a couple hundred rounds will make it filthy, and it is ieasy as pie to fire off a couple hundred rounds over the course of a standard kinetic mission day, which means you actually have to add "weapons maintenance" down time to your operation in addition to food and rest. You can clean an M4 thoroughly in under 10 minutes with the right tools, even quicker with the right chemical spray, but the worry is that you will get hit while peoples weapons are dissassembled.