Continuity mistake: When Margret puts The Gun belt around Frank's waist, it is twisted in the back. After the cut, it is righted. (00:02:30)
Continuity mistake: When Hawkeye passes by Frank on his way to Lt. Johnson's, he's holding a bottle of gin up near his waist. In the next shot, his arm is resting down by his side. (00:08:30)
Continuity mistake: When Hot Lips calls General Clayton to her tent, he closes the door twice. (00:11:45)
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