Every news article I could find on the subject, said AR-15. Their source appeared to be a congressman who disapproved (Jeff Duncan (R-S.C.)), and had no reason to sugar coat it.
Without any insider information, that's all I have to go on.
The Congressman actually seemed to be buying the anti-gun rhetoric about the AR-15. Based on his picture, he probably grew up after the draft ended. His Wikipedia page didn't mention any military service.... I guess it's possible that he doesn't know the difference.
BTW: Real Men are strong enough to control M-14A1s, and strong enough to carry enough "real" bullets to keep it fed, they don't need those little Mattel products.
Disclaimer: None of this has anything to do with the original topic, nor does it have any bearing on whether or not IRS agents have the right to bear arms.