Arthur, I don’t see anyone in this thread raising concerns about a service pistol having the same damage as a pulse rifle, or the magnum dealing more damage than a pulse rifle.
To me, that would be like a dagger in Forbidden Lands doing as much damage as a claymore, or a short sword dealing more damage. But hey, if it works for you, then fine.
As for Alien, not Aliens: I’ve been trying to fix this for a while now, but to me it just highlights one of the weaknesses of the Year Zero system. Damage is pretty much static, or close to it, and that works fine for near-peer combat—maybe not so much when there are significant differences between adversaries.
If we raise the damage of military weapons by a few points, they become more dangerous to the xenos—but then you would almost always kill human targets, and since PCs are human, that would make military weapons effectively unusable by NPCs.
Blade Runner had some pretty good ideas around critical hits, and when I got that, I tried to adapt it—but it ended up being a lot of work.
I don’t really understand why military weapons are included in the game at all. One approach would be to say that “Aliens” is outside the scope of this game and simply not include Colonial Marines or the M41A. In my gaming group, we played Chariot of the Gods, and when the PCs got hold of an M41A, I could see the excitement—this was an iconic sci-fi weapon, and the players had expectations about what would happen when they used it—only to be met with disappointment. It stripped 1 or 2 health from an abomination before running out of ammo.
The best success I’ve had with the Alien RPG has been with near-peer threats, either humans or Resident Evil–style Black Goo “zombies.”
But in the end, if the system doesn’t work for us, we’ll just move on. That’s what happened to my group.
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I understand the problem, and you're right, no one had discussed the marines' weapons. As I said earlier, we need to consider whether changes are necessary for our game table. Is there a real problem? If so, then make (some) changes with house rules. There's a very relevant point Fanhorn made earlier that explains (possibly) why all the weapons deal roughly the same damage. Fanhorn said all the characters have an average of 3-4 health points. That's true. Therefore, I don't think there can be a significant difference in weapon damage for that reason. Regarding your game table, you could increase the damage of the marines' weapons and add more Xenomorphs to balance the game at certain times. That's one solution, I think.
P.S. You're going to kill more xenomorphs and you're going to get closer to the "Aliens" model (hahahahah!).
P.S.2 If you upgrade the marines' weapons, for these specific cases, there's nothing stopping you from doubling (or multiplying by 1.5) the health points of each xenomorph to balance everything out.
I think I'm bringing you some good ideas here.
Arthur.