Considering that most pro gun people are also military friendly, I'm not sure that the militia requirement would seriously undermine gun ownership. (Again, illegal guns would stay there, and illegal gun owners are a different animal, many of which form de facto rebel armies -- gangs).
Conversely, if true conscription was the norm in the US, I seriously think that military adventurism would take a huge blow. It's one thing to send an army of volunteers to AfIraq, and another to send any battle aged, able bodied gun owners. Draft dodging and that volunteers have been enough in number have been key in keeping the US imperialist.
Of course, a positive would be that Americans would take a more serious look at the costs of binding alliances like Nato and the Asian alliances.