If its going to be a milsim - hosts need to provide the ammo and incorporate it into the sign up fee. Its almost a sure fire way of avoiding the sim turning into a skirmish.
Some of the most fun ive had were the games where i barely finished one mag. It was always a constant question wether an engagement was worth the ammo.
As a response to some of the above - the host makes the game, the players only contribute a more enjoyable expirience.
This upcoming year i know me and my guys are going to picking our major events more carefully.