What people are reporting about bump fire stocks being erratic and difficult to run is true when they are fired in a normal range type setting. That is when they are fired horizontally to the ground. However when bump firing either with a stock or without is done at an extreme downward angle (which this was) it's actually VERY easy to get very long and steady rate of fire. The reason for this is fairly simple. The trick to bump fire is pulling the weapon forward with an consistent force to keep the weapon bumping the trigger as it returns from recoil. When the weapon is pointed downward gravity does the work so the rifle is actually just bouncing up and down and activating the trigger.
This is the reason for the nice 500 RPM cyclic rate and the reason the bursts last about 10 seconds. 8-10 rounds per second empties a 100 hundred round Surefire mag in 10 seconds. Then the pause where the shooter takes a breather moves positions picks up a new rifle equipped the same way and repeats the process.
If the shooter actually had a belt fed he likely would have fired bursts lasting much longer than 10 seconds.
This was bump fire just as they are saying.
Good info, makes sense.
Figuring out WHY is the big question.