Sniper rifle. Any kind, as long as it is accurate. Most people ignore accuracy for amount of bullets, but killing one person out of one hundred without fail is more demoralizing and useful.
not too well known, but its a fully automatic shotgun holding up to 36 shells in a magazine, and can also fire FRAG-12's, a miniature grenade the size of a shotgun shell, and have a blast radius of 9ft. imagine 36 grenades being shot 2 ur house simultaneously. go google it