[BlueDragon]

There are double-drum magazines that hold 100 rnds of .223/5.56 Nato.

For .308/7.62 Nato AR-10 variants (as long as they are not Armalite) there are 50 rnd magazines available.

[Meet the New Boss]

Listening to the Drew Akioshi video on Twitter, I timed on a stopwatch three bursts of ten seconds each separated by significant breaks. Sounded like somewhere around 500 rounds per minute.

So it could have been 50 round magazines being changed out. Or, belt fed and letting it cool between bursts.

[Spktyr]

See immediately prior post - there are even higher cap drums out now for both and for others.