Deliberate mistake: With the show having lasted longer than the war, the producers apparently decided to turn back the clock to the very beginning of the War (June 25, 1950). To do this, it seems they have tried to make things as confusing as possible, intentionally mixing up facts, even when this openly contradicts the facts as presented in earlier episodes. For example, both B.J. and Potter arrived in or about mid 1952, but now are in the O.R. when the Chinese launch their massive offensive on 1st Nov. 1950, almost two years earlier. Trapper and Blake, who were there for the Army Navy Game which would have occurred Nov. 25 1950, are not. Many of the supporting clips also are too early or late for the events being discussed. For example, Winston Churchill's speech must have been before August 28 when the first British troops started arriving, likely also before the Canadians in July 1950, but the first MASH Unit was not set up in Korea until September 1950. Truman's speech to University of Wyoming students is actually from May 9 1950, a month and a half before the beginning of the conflict and 5 months before the first MASH was established, but the clip about his "Whistle Stop Tour" must be from the 1948 campaign. MacArthur's speech, on the other hand, is in response to the ticker tape parade of April 22 1951 when he was replaced.
Factual error: Nearing the end of the show, we are told that the Chinese have become involved in the conflict and have attacked with some 300,000 soldiers. This apparently refers to massive attacks by the Chinese starting on November 1, 1950. Although the numbers are correct, the US/UN forces were unaware of the numbers. As late as November 6, the Far East Command continued to insist that there were no more than 34,500 Chinese communist soldiers in country. http://www.history.army.mil/brochures/kw-chinter/chinter.htm.
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