Well, she didn't gave any specifics on the routes when she tried to hire Mark, and he said that getting those supplies would be a condition for accepting the job, she agreed.
Also, it's just an LMG, you may as well need it against fiends, legion or nearly any other enemy short of a radroach, none of us is supposed to be the courier, killing a whole legion squad with a varmint rifle. And unless we play scavenge for supplies all the time, in wich case we would have to "roll" for loot and have a loot list elaborated from before, otherwise nothing keeps you from finding a machine gun later just lying there if you are just going to create your own loot whenever you want, I don't see what's wrong with having the gear from the beginning, we are not trying to level up or farm for items like it's a mmorpg.
Besides, I don't see what's wrong with encountering a deathclaw head on, why wait? It's not like we have to be a "better level" to face one and survive, at the beginning of the story or at the end we will all be the "same level" unless we are playing with stats, wich we are not. This kind of game focuses more on the story unless we are using a system.
That's why I didn't want to go into detail with the items at first, no one is going to be counting how many bottles of water you drink every day and how fast you dehydrate and then tell you you have died of dehydration or hunger. It was just for explaining that as least as far as Carib and Mark were concerned they were going to go in as a fire team and were going to take better supplies as something related to the story, I didn't expect we were going to start counting the availability of cram.
So forget all the details about the items, we are a fire team, we have one support weapon ont he whole team in case we run into trouble, and we have food and water covered for the duration (although we could have just assumed that but hey, I just wanted to introduce that as a story element). No one is going to count the ammo, we have "enough" for now at least, if we need to run out of ammo later in the story, for purposes of the story only, we can work around that later.
And it doesn't HAVE to be a deathclaw, it was a concern Mark had before in the story, but we may end up not using the LMG at all and it ending up just being dead weight. Or maybe we'll need it... It doesn't mean anything. If you want to tell Mark he is overreacting for getting an LMG because that's what your character thinks, then do so in the IC.