Used to do the same when in the cadets. Most mags would hold in place if you just kept it against the catch point.
That reminds me, how many times did someone try to catch you out with a blank round in the breech hoping you would not check the weapon and pull the trigger.
On one squadron I was with a WO used to do that and finally it happened, idiot cadet pulls trigger and "BANG". Right in the middle of a visit by the local mayor. Just as well it was in the 60's and not in the 10's.