Magnum - Originally, a large wine bottle. In shooting, it refers to a cartridge of exceptional size or power. First applied to large bottleneck cartridge, hence the name. Magnum is more of a marketing term than a technical one.


Mean Radius (MR) - The average radius of a group of shots from the center of the group. Another method of recording accuracy, MR is commonly seen in military contract specifications for accuracy. Extreme spread is more commonly used to measure accuracy in commercial ammo manufacturing and hobby shooting.


Meplat - The flat or blunt end of the nose of a bullet. The French pronunciation "meh-PLAY" is seldom used.


Mercuric Primer - A primer in which the primary initiator is mercury fulminate. These primers have been obsolete since the advent of metallic cartridge reloading over a century ago. On firing, the compounds release minute amounts of metallic mercury. Mercury attacks the cartridge case making it brittle and thus unsuitable for further loading.


Metal Case (MC) - Also Full Patched (FP) or Full Metal Jacketed (FMJ). A type of bullet in which the core is completely encased in jacket material, except for an opening on the base. Standard military bullet type.


Metal Fouling - Bullet jacket material deposited in bore due to friction. More common in very high-velocity rifle cartridge, metal fouling must be removed to prevent corrosion due to the electrolytic action of the copper against the steel barrel. Metal fouling can also result from a rough bore at any velocity.


Micrometer - A measuring instrument with a fine screw adjustment for measuring very small distances. Usually calibrated to read in increments of 0.001" or 0.0001".


Mid-range Trajectory (MRT) - The highest vertical distance of a bullet above the line of sight at a point approximately halfway from muzzle to target or point of aim. The MRT is varies with the zero range for a given load. Also called the maximum ordinate.


Minute-of-angle (MOA) - A unit of angular measurement equal to 1/60th of a degree. Although usually approximated as one inch per 100 yards, it is actually equal to 1.047" per 100 yards.


Misfire - Complete failure of a cartridge to discharge after the primer is struck by the firing pin.


Mould Blocks - Two "mirror twin" pieces of metal having a bullet-shaped cavity in which lead bullets are cast from molten lead.


Mushroom - The ability or capacity of a bullet to increase its diameter upon impact with animal tissue. The name comes from the desired shape after expansion.


Muzzle - The front end of a barrel. The point at which a projectile leaves the barrel.


Muzzle Blast - The compression of air caused by hot, high-pressure powder gases jetting from the muzzle of a gun.


Muzzle Brake - A deflector fitted to a gun muzzle to deflect exiting gases. Usually used to reduce recoil by redirecting the jet effect of muzzle blast. Also called a recoil compensator, or comp for short. See compensator.


Muzzle Energy (ME) - The energy of a bullet at the muzzle. At this point a bullet's energy is highest. See energy.


Muzzle Pressure - Gas pressure in the barrel at the muzzle at the instant the bullet leaves the muzzle.


Muzzle Velocity (MV) - See velocity.