I'm still not convinced that it's entirely illogical: a little un-necessary, perhaps, but better safe than sorry.
Maybe you're right. Thinking about it, in Flight 714 it turns out that there are in fact aliens buzzing around our solar system, so in terms of the reality of the Tintin books as a whole, I suppose it's not out of the question that Tintin and co would encounter beings on the moon.
Possibly, but obviously some other authority in the space programme thought that weapons were a good idea and supplied them, as they are patently there on the Moon!
Good point. The Syldavian government does seem a bit militaristic in Destination Moon, and even more so in the immediately post-Moon-adventure, The Calculus Affair. Maybe they insisted that guns be carried on the mission (and on any lunar cave exploration trips), regardless of Calculus's scientific beliefs and pacifist tendencies.
Also, I have thought of a further use for guns on the mission as a whole, if not for the actual cave scene:
If you do what Haddock does on the trip to the moon, and leave the spaceship in the middle of space and float off towards an asteroid, I think you could use a gun to propel yourself back to the ship, by firing it in the opposite direction you wanted to go in. Mind you, that relies on you being sober enough to take the gun with you, and if you were that sober, you probably wouldn't leave the rocket in the first place.
Still, as a primitive space-walk propulsion sytstem, for making repairs to the outside of the rocket en route, I suppose a gun would be better than nothing.