The Sniper Rifle is a weapon included in anon-hl2 that was ultimately cut from the final release.
The Sniper Rifle is a powerful weapon, able to kill most enemies in a single shot. It even has a scope that allows for further accuracy when firing, making it the perfect weapon for long-range situations. It is however hindered by a view-bobbing amount of recoil and needing spend several seconds reloading after every shot, making it a very uncertain bet in closer ranges.
The weapon was ultimately cut, in favor of the projectile based Resistance Crossbow. However, it still is in Half-Life 2: Survivor, in an altered form.
It appears to be loosely based on the real life LAR Grizzly Big Boar Rifle and takes some inspiration from said gun.