Continuity mistake: When Radar goes to get the lamb, he pulls out a hypo to put the lamb to sleep for shipping. When the shot cuts to the lamb, you can see both of Radar's hands on the lamb's face; no hypo in sight.
Revealing mistake: The sound of crew making an explosion with their mouth as a cue can be heard just before the actual audio of an explosion plays.
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