The G150 is a rifle specifically assembled by and for the Swiss P-26 organization: a very secretive stay-behind group intended to fight foreign occupiers of Switzerland. It was one of a series of such organizations that began with a concern during World War Two the Germany might invade, and continued during the Cold War with the threat of Soviet occupation in the aftermath of nuclear war. The P-26 group specifically was formed in 1981, and disbanded in 1991 under a cloud of controversy over its political leanings.

P-26 was armed with an assortment of weapons ideal for guerrilla warfare, including P210 pistols and suppressed MP5 submachine guns. The G150 rifle was intended to be a very quiet rifle for destroying enemy materiel like radar systems, fuel tanks, parked aircraft, and the like. About 250 were made using commercial JP Sauer actions, SIG 540 like pistol grips and folding stocks, and very large two-part suppressors. They were chambered for the .41 Remington Magnum revolver cartridge, loaded with a 408-grain subsonic bullet. The scopes were adjustable from 4-6 power (yes, 4-6: it's a weird choice) and had BDC elevation turrets adjustable out to 200 meters.

Only three G150 rifles are known today, although the remainder may still be in some deep military storage in Switzerland. Many thanks to the anonymous viewer who arranged access to this one for me to film! To see another perspective on one of the other known examples, I recommend Bloke on the Range's video:
https://youtu.be/VHopVhV1vWA