There aren't enough troops currently in the CONUS to implement martial law, except maybe in areas of natural disasters, and even then, it is a "onesy-twosey" response. That ans there are areas of the US where the Army may actually be out-gunned!
Both statements are true, except we have more tanks, helicopters and artillery. But we'd never be able to take every individual firearm away. And thank God. Standing armies are a dangerous thing anyway. I'd never ever want to have more soldiers than armed citizens.