Big Bore Compendium – .600 Overkill

OLYMPUS DIGITAL CAMERAThe 600 Overkill was designed by scientist Robert Garnick of Las Vegas, Nevada. It was intended to take full advantage of the dimensions of the CZ550 action in order to fit into that action the largest cartridge that could be accommodated. With moderate alterations, a CZ550 can be made to accommodate 2 cartridges of 600 Overkill in the magazine plus one in the chamber. Using brass solids bullets, the 600 Overkill will penetrate 6 feet into an oak log. Even elephant skulls are not as dense as oak, assuring that the 600 Overkill pushing 900 grain solid bullets at 2400 f.p.s. will offer excellent penetration on any game.

The 600 Overkill case originated from the 600 Nitro Express. The rim was removed from the 600 Nitro Express and a belt was added for headspacing purposes. A 0.640″ rebated rim was cut on the case – the same size used by the .505 Gibbs. The case walls were left straight for a 3.0 inch case and a 3.75 inch cartridge overall length. Custom 3 groove barrels with 1:16 twist are made by Pac-Nor Barrels of Brookings, Oregon. A 3 groove barrel with a 1:16″ twist was chosen to keep pressures low and permit the cartridge to achieve higher velocities than would be otherwise possible. This scheme seems to have worked because with the same powder charge and bullet, the 600 Overkill achieves about 75 f.p.s. more velocity than the 600 Nitro Express.

15The 600 Overkill has the ability to push a 900 grain bullet at more than 2400 f.p.s. if the shooter can withstand the recoil and maintain his grip on the gun. At that level of power, as the bullet spins down the barrel, the rifle is torqued in the opposite direction, tending to twist the forearm of the rifle out of the shooter’s hand. Brass for the 600 Overkill is available on a special order basis from Dieter Horneber ( of Germany. Rifles may be ordered from American Hunting Rifles for not much more than US$3000. For the big bore fan who needs the biggest affordable big bore, the 600 Overkill based on a CZ550 action is for him.

Example loads:

Cartridge Powder Primer Bullet Velocity Notes
600 Overkill 158.0 grain IMR7828 F215 900 grain Woodleigh FMJ 2150 f.p.s. Controllable recoil when fired in a 14 pound rifle with 3 mercury recoil reducers and a muzzle brake.
175 grain H414 2400 f.p.s. The forearm is difficult to hold onto at this velocity due to torque created by the bullet spinning down the bore.