Regarding M4 double feeding, here's the FM solution:
U.S. Army Field Manual, FM 3-22.9, Chapter 3
Troubleshooting and destruction
[..]
3-2. Malfunctions
a. Failure to Feed, Chamber, or Lock. A malfunction can occur when loading the rifle or during the cycle of operation. Once the magazine has been loaded into the rifle, the forward movement of the bolt carrier group could lack enough force (generated by the expansion of the action spring) to feed, chamber, or lock the bolt (
Figure 3-1).
[
Figure 3-1 Failure to feed, chamber, or lock, showing what I believe to be two M16A2 uppers with the lower one having a round injected into the chamber. Between them list reads]:
- Failure to feed first round.
- Bold carrier not fully closed.
- A double feed of two live rounds.
[...]
(2) Corrective Action
Applying immediate action usually corrects the malfunction. To avoid the risk of further jamming, the firer should watch for ejection of a cartridge and ensure that the upper receiver is free of any loose rounds. If immediate action fails to clear the malfunction, remedial action must be taken. The carrier should not be forced. If resistance is encountered, which can occur with an unserviceable round, the bolt should be locked to the rear, the magazine removed, and the malfunction cleared. For example, a bolt override is when a cartridge has wedged itself between the bolt and charging handle. The best way to correct this problem is by-
- Ensuring the charging handle is pushed forward and locked in place.
- Securing the rifle and pulling the bolt to the rear until the bolt seats completely into the buffer well.
- Turning the rifle upright and allowing the overridden cartridge to fall out.
The immediate action drill is listed on that same page, although I'm positive you already know it.
Just my .02
Martin