As Jak said.
Have all the mags you'll need on yourself.
As with my F2000, it'll take other M4 mags but some M4's wont take the G&G F2000 magazine.
Normally a little bit of filing on the magazine itself or the mag well will be enough to make them fit.
I personally wouldn't lend a stranger any of my mags so mag swapping is something left to teams, whom normally field the same weaponry.