Continuity mistake: While Colonel Potter is painting the portrait of Charles, the portable military field desks behind Potter are all open - note the front doors are flipped down, but in the closeup of Charles holding the painting of himself the doors are folded up. And the books, etc. atop the field desks change as well.
Continuity mistake: While Charles is reading the letter about his nephew being discharged because of fainting spells, Klinger feigns a fainting spell falling forward onto Hawkeye's cot, and his body lies lopsided with his leg hanging off, but when it cuts to the next shot Klinger's perfectly positioned on the cot. Also, note the mail Klinger tossed earlier onto Hawkeye's cot, near his pillow, is now neatly tucked under the blanket towards the foot of the cot.
Continuity mistake: In the last O.R. of the episode, Winchester is wearing a surgical gown with the same blood stain pattern as in the previous episode. This also would be impossible given the cleaning procedures.
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