6P62

6P62 (6П62) is the GRAU designation of a Russian hand-held fully automatic anti-materiel rifle chambered for the 12.7 x 108 mm round. At 100 meters it can penetrate up to 20 mm of RHA steel.
Only one similar weapon was ever constructed, the KPB-12.7 (КПБ-12,7). It was intended as a hand-held machine gun. Some sources state the weapon was uncontrollable when shoulder fired in full-auto mode. The recoil-reducing system allowed this weapon to be fired from the shoulder, but it still had an insufficient reliability and an insufficient accuracy in single shots, and accurate bursts were also all but impossible from such large caliber but lightweight weapon with a limited magazine capacity.
Some of these drawbacks are apparently inherited by the 6P62, since it uses the same powerful cartridge on a similar, though little bit heavier and stabilized with a powerful muzzle brake, platform. The low magazine capacity with such weapons is apparently a constant.
Having all these downsides it would be difficult to presume such weapons can be truly called a hand-held ones.

KPB-12.7 (КПБ-12,7)

Operation:
Gas-operated

Cartridge:
12.7 x 108 mm

Weight:
15 kg (without rounds or additional sights) 3.5 kg (weight of the loaded magazine)

Length:
1200 mm

Magazine Capacity:
14-round

Feed system:
detachable box magazine

Sights:
Iron sights : 100-1000 meters (optical sights as an option)

Rate of fire:
400-500 rounds/min

Effective range:
1000 meters

Muzzle Velocity:
620-645 m/s