The hit may be stopped by the armor. If effective, the hit may be fatal or not: due to better emergency medicine in the army, they will save most of their non-immediate casualties. Irregulars have historically lost over half of their wounded.
The direction of the sniper would be indicated by the backsplatter from the wound. In case of a miss, the projectile trace in earth or trees would just about pinpoint the location of the shooter. At which point, the infantry would use smoke to obscure themselves from the shooter, make his egress perilous with suppressive fire and get close enough to exact retribution. The civilian shooter would have no land mines to disrupt their progress.
A really good shooter can make 500 yards hits on bullseye targets. Can he do as well on camouflaged foes who move, use cover and can put literally a hundred times his rate of fire with their squad MG or SAW? An individual rifleman might be limited to 300 yards, but a SAW with a scope fired from a bipod can reach out more than twice as far.
Assuming that the sniper can manage to retain his stand-off distance, what can he do about air or artillery response. Insurgents world-wide have to brave close combat to get away from the firepower available to the regulars with one radio call. Against well-designed sniper hides that cannot be reached by artillery or airpower, short-range rockets would be used. The hide would have to escape detection in visible, near and far infrared ranges to remain secure.
Multiple snipers might do better, but the regulars can just continue using smoke to remain safe from long shots while encircling the whole area and plastering it with mortars or just mining the perimeter and leaving it at that.
I am a great fan of rifle marksmanship. But we shouldn’t overestimate its value in warfare. Unsupported by regular troops, most snipers die quickly. Most hunters may be marksmen, but they aren’t even snipers — that skill set goes far beyond the basics of fieldcraft and marksmanship required to bring down deer.
For that reason, the reliance on armed response indicates a loss for the side forced to fight as insurgents. The kind of expedients required for a successful guerrilla campaign tend to warp all participants out of recognition as the “forces of good”. So our best bet is political proselytizing and raising the next generation to love freedom, and to respect the freedoms of others. The opium pipe dreams of the “restoration of the Republic” through another revolution are best left for those who don’t much value a connection to reality.