A sniper was able to cap off 2-3 enemy troops before his location was found and serious counter-fire ensued (at least in Vietnam). THISISME, I don't doubt your marksmenship, but in real combat snipers don't tend to last very long unless they are very, very good and only fire one shot. After you fired your initial shots killing a few men, they would zero in on your location, call in artillery or close-air, advance using natural cover (arroyos, ravines, etc.) and proceed to use the infantry's primary weapon, the hand grenade. In WWII, Korea, and Vietnam the squad's LMG was used to provide cover fire while the squad's best grenade chuckers ran forward to lob a dozen or so grenades at the enemy, this included counter-attacks against snipers. In Wai we used a 106mm recoil-less rifle mounted on a mule to take out snipers, tanks with flame throwers were also very useful.
I think something that we are all forgetting is that infantry rarely advances alone in combat. They have tanks with them. One of the posters already said something about running gun fights and booby traps, he is more right than any of us. That is what the VC did and they were fairly successful at keeping us on guard all the time, which is very tiring. Land mines are also relatively easy to make, even anti-tank mines don't require that much engineering skill.
And, I think the possibility of an invasion is far more unlikely than an attack on gun owners by the Feds using military hardware (remember Ruby Ridge and Waco?). Hurling insults ("you must be a piss poor shot") over a nearly impossible scenario is kind of pointless, don't you think?