I once read that the largest standing army on the planet is in the U.S. on the opening day of deer season. Not sure if that's factually accurate but it does sound formidable.
Consequently the best firearm a guerrilla fighter can own is a high powered hunting rifle with a good scope. Also, the average hunting rifle sold in the U.S. is better than the sniper rifles we recovered in Iraq. Nothing is more demoralizing to an invading force than snipers, except perhaps anti-personnel mines.
Are all those rifle owners regular active hunters? How many of the hunters possess military caliber marksmanship? Stalking a deer through a forest and sniping members of an invading enemy both involve shooting rifles at moving targets. Being holed up in a sniper's nest during a shooting war and taking out human targets that are aware of your presence and either shooting back directly or trying to relay your position to better placed or equipped comrades and all the while moving fast and erratically would be extremely challenging for non military trained folk, hunters and otherwise.
More significantly, which countries do you think are in a position to attack the lower 48 states? Canada and Mexico. Can't see that happening any time soon. Sadly, an American on American conflict seems far more likely than a foreign invasion, but even that would require a a civil society and social order break down of unprecedented scale.
| Being holed up in a sniper's nest during a shooting war and taking out human targets that are aware of your presence and either shooting back directly or trying to relay your position to better placed or equipped comrades and all the while moving fast and erratically"
Consequently the best firearm a guerrilla fighter can own is a high powered hunting rifle with a good scope. Also, the average hunting rifle sold in the U.S. is better than the sniper rifles we recovered in Iraq. Nothing is more demoralizing to an invading force than snipers, except perhaps anti-personnel mines.