Proud ignorance and gun control
I was musing on this very topic right before Obama came out with his "fully automatic" comment. (I recall some initial speculation that Obama might have been casually or inadvertently disclosing a previously unknown fact of the Newtown case, but I think we're safely able to assume ignorant folly by now.)
Interesting point about the hypocrisy of the general liberal sneer about know-nothing scolds. I also suspect much of the apparent ignorance displayed by gun-control leaders is deliberate, an attempt to confuse the issues. Automatic weapon = semiautomatic weapon = military hardware, and all that. A confused public is a frightened public. The gun control zealots think they'll reap political dividends from getting voters who don't know much about firearms programmed to think "semiautomatic weapons" are implements of mass destruction that any sensible person would want to keep off the streets. Language control is thought control.
I think the other factor in the often astounding ignorance displayed by the gun-control crowd is that it's fashionable. Their constituency is largely made up of people who are not gun owners, much less firearms enthusiasts. They get a lot of money and support from the upscale suburban crowd. Displaying a thorough knowledge of guns would alienate those people. They really don't want their movement leaders talking like correspondents on the Guns & Ammo letters to the editor page.