I could see magazine restrictions possibly passing, but not ammo limits. Even a 100 round at a time limit would be highly intrusive and easily circumvented, yet it wouldn't really do anything since these sorts of out-of-the ordinary sprees generally seen over 100 rounds actually used. The tragedy in Norway is the only one I can think of off hand that used over 100 rounds.
I believe Manchin was arguing for a sales limit of 30 rounds at one time. Even that seems unnecessarily permissive to me--what is gained by allowing more than maybe 5 or 10 rounds at once? While I agree that a really dedicated person/organization could just go around to many different gun stores or purchase ammunition over a period of months or years, it does at least make short-term planning for these sorts of things more difficult.