The Denel-Mecham NTW 20mm Rifle
The South African Denel-Mechem NTW-20mm Anti-Material Rifle qualifies quite admirably for the title of Worlds Most Powerful Sniper Rifle, and I don't think that this will be disputed.
This rifle is a serious "take no prisoners" weapon !
The Denel-Mecham rifle fires massive 20mm cannon rounds as more favored by helicopter gunships, fighter aircraft, warships and armored vehicles. Since the 1930's up until recently, the 20mm round has never been deemed suitable by the military as ammunition for use in a rifle, as the rifle required to fire it would be fantastically heavy and way too cumbersome for any infantry soldier to carry. Plus the fact that the 20mm cartridge produces such excessive recoil that no-one would withstand the force.