I might let my players start with a minigun (even though I'm pretty sure it is R/R). As long as there is a logic behind how it is in their hands.
Given I use
Graebarde's 5th Mech Reorganization document, I would say that the gun was scavenged from one of the four vehicles that have them in the Mad Max battalion. I might even give them one of the 4 x five tons ('GRIM REAPER' 'SHOGUN' 'ACE OF SPADES', 'VALKYRIE') he mentions.
Probably the only way I would have a minigun on a HMMWV before battle of Kalisz is as some sort of command escort. One of the weapons guides discusses miniguns and says they are a prize sought after by command tanks. Therefore I see their placement primairly around those who have a little clout.
As far as ammo roll the dice (or just say you did
). Remember no plan survives first contact with the enemy, so the group will have had a few curves thrown at them in their escape.