Well, furthermore, those weapons were totally useless against the alien, itself. Yes, they're guns, but all they really give you is a false sense of security. I suspect in Covenant, it will be more akin to Aliens, with the guns being potentially able to at least wound the beasts; but being in outer space, the last thing you'd want to do on board a space ship is start a firefight. Doesn't, at one point
a fire start on board the ship, killing at least one crew member?
It makes me thing they'll be using the guns more on the surface of the planet, or as a last, desperate defense.
On the planet, I think the beasts will be different than the xenos of the past that we've seen, so I'm not sure how effective the weapons will be. Obviously they can't be very effective or there'd be no threat--unless Ridley pulls an Aliens:
In Aliens, Cameron's approach was to make the guns lethal to the aliens, but even then, they were ineffective against the hive, as a whole (ergo, Ripley's suggestion to take off and nuke the site from orbit); furthermore, Cameron even made the weapons' leathality a disadvantage to the marines, with the potential damaging of the cooling system of the atmospheric processor potentially resulting in a thermonuclear explosion. Adios, muchachos. He essentially said, in an old interview, that Zulu Dawn can be just as exciting as a slasher film with one guy running around the house, chasing someone with a knife. For all intents and purposes, his approach worked. Yes, those guns were effective against the aliens, individually, but even then, it wasn't safe to kill them--just ask Drake or Hicks. The weapons, even with their lethal effectiveness, still reaffirmed Parker's statement in Alien: "It's got a wonderful defense mechanism; you don't dare kill it." On top of that, in Aliens, even if you did kill a few of the beasts (or even most of them) the surviving members of the hive would do you in, assuming you didn't die from injuries sustained in the battle (acid, or, in Frost's case, friendly fire--pun intended).