@Caligura To be fair, it's somewhat implied that the primary suspect might've given the rest of the cult some form of training; she's noted to be ex-intelligence, and like you said it's possible that it might all be a psyop. IRL, Bonnie Parker - of the infamous Bonnie and Clyde duo - was noted to be a pretty good shot with a BAR, so it is possible for a woman to carry a stronger gun... but as you mentioned, it would take quite a bit of training. Realistically speaking, it would've made more sense for the cultists to be carrying something with a lighter weight, like pistols or carbines, but given that the mission is later in the game, it's likely the devs just wanted to increase the difficulty somehow.
Also, RoN is already pretty over-the-top - the second mission is a SWATTING incident with a furry streamer that results in the police getting into a gunfight with an entire group of armed assailants and uncovering both an illegal crypto farm and a child porn ring. This type of stuff isn't surprising.