They are loading spare magazines with ammo, and arming grenades. The number 36 grenades came in two boxes, one with fuses, and one with the bodies of the dozen grenades, and you had to arm them, and cut the fuses to a 3 second timing, so the gerries couldn't throw them back at you.
A good thrower can get one out to about 50 feet and anywhere within 10 feet it can kill you, if you are in the open. In a room, it is much more deadly.
