I know that some players are concerned about the limited ammo (200 rounds) for three missions. While it may seem inconceivable that 200 rounds would suffice for the event, this rule is designed to make the missions a challenge.
During the first mission, each team will have about 20 -24 players. Teams will have to decide just how agressively to pursue their objective vs ammo conservation. Players in the game have the luxury of knowing that a zombie apocalyse is waiting in the next two missions instead of just having focus on the rescue of trapped hostages or fighting off a military response team.
By limiting ammo to such an extreme, combatants are forced to rely on individual tactics, marksmanship, team work, and developing an overall survival strategy. Those who choose to barricade and "hold out" will quickly find that what's left of your 200 rounds after mission 1 will result in you becoming an additional entree at the all you can eat zombie buffet.
Those who have played at our zombie games in the past have seen that 100 bb's does actually go a long way. I've seen just a couple of guys survive against a horde of 50 with one mag each for close to 10 minutes with good team work, communication, and most importantly keeping a cool head.
Remember that just because you are out of ammo doesn't mean that you are out of the fight. Help your team as best you can until you find a resupply. If all else fails, sacrifice yourself to the hungry mob and buy your team members a few precious seconds so that they can make it to the chopper.