The notion that you need guns in civilian population to have an effective army is funny. Maybe if your army is a bunch of amateur militia types then yes, but we expect professional soldiers to be trained and again, the professional military equipment has no place on the streets in the hands of civilians which renders the argument void.
I don't think the argument is as simple as needing guns in the civilian population in order to have an effective army.
Insofar as it's been presented to me the argument is more along the lines of a US population that has established a culture of standing up against governmental thuggery to a point that its citizenry is willing to put itself at risk in order to further what is in their mind a better way of governing. This willingness includes a willingness to fight and die for other peoples under threat. It happens to be the case that gun ownership has played a role in the construction of this culture. As the argument goes, without personal gun ownership its uncertain that the citizenry of the USA would have evolved the culture that led to this willingness, which in turn led the US citizenry to help defeat horrendous governmental aggression of the 20th Century.