The original assault rifle
(matrix.gvid.tv)https://matrix.gvid.tv/c/Libertarian/EYtGrBCWDm.jpgThey wanted YOU to have more advanced gun technology than the government. Read the text of the second Amendment. It isn't just about your right to have guns. It's also about the government obtaining access to guns exclusively through the citizens.
They were not unfamiliar with military's obtaining their own armaments. That's how the British army worked. The 2nd Amendment indirectly specifies that if the government wants access to guns they should obtain them through your permission, not the other way around, via the citizens' nearly exclusive possession.
When liberals say the Founding Fathers never intended you to have AR15s its even more interpretable that they didn't intend for the government to have them.