You're going in circles here. You're defending the notion of the gun having realistic weights by saying they have unrealistic weights because game. But this is very OT...
That's guns, not ammunition. Guns (Rifle and Pistol and Plasma weapons and Rockets and Rocket Launcher) seem to sit at the point of weight scale where there's still weight in pounds. Lower end of scale is limited because of the throwing calculation - which makes bizarre grenade and magazine weight and weights on heavier items like corpses are in some different units due how encumbrance works - with corpse weight in pounds, carrying a corpse would leave character with 7 action points or something like that.
However what would go against adding external weapons damage (potentially, I'm not saying I'm against the idea): we don't know if the UFOs sustain any external damage at all; if they were simply shot until stopped working, they'd go straight down and smash into a million pieces... Maybe the human weapons merely cause their power systems/forcefields to overload and force emergency landing... that sometimes leads to power source overload and explosion. That'd explain why they're either seemingly undamaged or blown to pieces, with nothing in between.
If aliens had forcefields, landed alien craft would have intact forcefield generators that could be researched. I agree that UFO systems certainly aren't completely disabled though.
Maybe aliens would emergency land by themselves when their craft is sufficiently ventilated? Larger alien craft probably could take quite a battering before that. Crash landed Battleship could literally have a large part of the outer hull stripped off it. It could look like a water battleship after taking 15 torpedo hits except without sinking.