Continuity mistake: When the red head woman is playing the piano, it's a double. In the close up her hair is back and you can see her face. In the wide shots her hair covers her face so as to conceal the switch.
Continuity mistake: As Potter pours the drinks for himself and Brandy, the glasses have enough for a sip. In the close up, they're a quarter to half full.
Continuity mistake: In Potter's tent, he holds the door for Brandy to enter. In the close up, you see him drop his left hand to close the door after she passes him and inside. After the angle change, his hand is back on the door, and she's just even with him walking forward.
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