M72 Gauss Rifle (Fallout 2)

The M72 Gauss Rifle is a weapon of German design. It uses an electromagnetic field to propel ammunition at high velocity. Its range, accuracy and penetrating power are almost unparalleled.

Location
It can be found in Marc's inventory in the bar on the tanker in San Francisco. You can obtain it either by killing Marc and looting his body or by pickpocketing him. One of the black haired female vagrants on the tanker has this gun in her possession as well. Red 888 in San Francisco will stock Gauss Rifles on occasion (along with Gauss Pistols and 2mm EC).

The Oil Rig has a few in some of the ammo crates.

Lastly, if you come across a random encounter with a cave that has something in them, the cave may be full of very well armed robbers. If your level is 19 or higher, the robbers will be equipped with combat armor, and some will be armed with this rifle (less in number compared to other weapons). Other weapons these robbers use include Turbo Plasma Rifle, YK42B Pulse Rifle, FN FAL, H&K G11E and Mega Power Fist.

The only place you can definitely find this weapon is the Sheriff in NCR. You cannot pickpocket it (probably has that equipped) therefore you will have to kill him to obtain it. Characters with high enough sneak skill can equip a weapon like the bozar while sneaking, get really close to the sheriff and kill him in a single shot. Keep hiding and than grab the rifle from his dead body. Also, you will get 150 rounds for the rifle with this method.