Other mistake: After pulling out of the camp to head for the cave, The Bus is shown from several angles. Even though it's loaded, the shots of it travelling are re-used shots from an earlier episode, and the place it stops is a re-used shot from the episode "Rainbow Bridge"
Preventative Medicine - S7-E22
Other mistake: During the duration of the episode a wounded person is addressed as Corporal North but during the credits he is called Private North.
Answer: Even an Army field hospital would have SOME weapons for self-defense if the enemy attacked. What's meant is that it's not a combat outfit, and therefore wouldn't have enough weapons to actually attack anyone. (It's also mentioned a few times that the Officer of the Watch is supposed to carry a pistol at all times, but Hawkeye refuses to do it when he's in charge and Colonels Blake and Potter don't enforce the rule).
Captain Defenestrator