Another thing you should know.
Usually with most mags the last 3-4 BB's will not feed. They're going to fall out just because the spring in the mag usually doesn't have enough power to push out those last few BB's into the gun. So in reality if you're using a real cap you're only going to get 26-27 BB's out of it.
Also some milsim games use a "point system" for mag loadouts where you're alloted 10 or so points and a real = 1 point, low = 2 points, mid = 4 points or something and high = 10 points. You're allowed to have any combination that gets you to 10 points whether it be 10 real-caps or 5 low caps or a combination like 2 mids and a low.
OP: This guy is an awesome cameraman and has A LOT of airsoft videos of larger games in the UK: