My headcanon is that revolver is intentionally underpowered (maybe some weak-ass 22lr cartridge) so it wouldn't pierce through a hull.
Same with a shotgun - it's a riot control weapon, so it shoots rubber instead of slugs/buckshot.
IIRC the standard canon was that the xeno is essentially bulletproof against all but the largest calibers and/or armor-piercing rounds. Of course there are weak spots in the armor (inside of the mouth, key parts of the 'face') where any half-capable firearm can do damage, but generally it's not going to be bothered much by 'standard' guns.
The ammo onboard the Sevastopol is probably flechette rounds or something similar to specifically prevent people from blowing out the hull.
I'd pay good money to see a xeno go up against one of those transit trams at full speed, though.