Start by defining what your professor considers "reputable" . If he's going to "rule out" such sources as the NRA, the FBI UCR etc then you're paddling upstream against a rapids. FWIW, the "crime rate" - per several industry and fed sources - for legally owned/registered full auto weapons is vanishingly small, ( Like the third digit after two zeros and a decimal point !).
basically it can't be something written from the point of view of someone who has little knowledge of the subject (newspaper) or someone who is not qualified (other students, international articles) etc. etc.
And yea, I had seen somewhere that the amount of murders committed with legally owne full-auto firearms since 1934 was 2...One by a police officer, and one by a man who didn't even know his weapon was full-auto capable..