Before joining the army, the only weapons I had ever fired were BB Guns, bow and arrows and slingshots. With only the Army's standard training, I managed to fire expert with every weapon they put in my hands. In qualifications I missed one target out of 70 with M16, hit all targets with 45 pistol, 50 cal. machine gun, M60 machine gun, M203 grenade launcher, TOW anti-tank, "personnel launched" hand grenade and my mortar crew also came in with top scores (I was loader, computer and FO at different times). </brag>

Paying attention to the instruction and not having any bad habits to break did the trick.

Hitting a human sized target at 100 meters is easily done by most human type persons with any reasonably accurate rifle.