(let me caveat with: I don’t like this idea for a variety of reasons)
What’s the biggest difference between stray-19-year-old buying a gun, and 19-year-old soldier carrying around a gun?
Lessee… a few hundred hours of training, a highly-trained chain of command with multiple layers of oversight, limited access to ammunition and weapons locked in a centralized controlled location until needed (they don’t sleep with guns & ammo in the barracks at Ft Hood), and a clear and dedicated purpose for why they are carrying their arms whenever they have them (on a mission, headed to the range, etc).
So the key difference? The entire ecosystem of responsibility in which the soldier exists that the civilian never will. Ignore that important distinction at your own rhetorical peril.