Maximum Effective Range: The maximum distance a weapon is expected to be accurate and achieve the desired effect...
http://www.ask.com/web?q=what+is+the...rc=0&o=0&l=dir

There are a few others but that is the basic definitition. When talking about battle rifles vs sniper vs hunting rifles it is a little more difficult to define let alone predict from shooter to shooter.

Example 1: When talking about an M16A1 the advertised MER is 460m. We all know the rifle will shoot further than that and hit targets beyond that. (I have hit targets out to 600m with that weapon system.) When I was in the USMC, I had to shoot them at 500m to qualify. I hit 10/10 at 500m the 1 time I had to qualify on the west coast. (East coast ranges were measured in yds when I was in)

In this case the MER was 460m because that is the maximum range a Soldier (not a Marine) was expected to get reliable hits and kill/remove enemy from the fight.

Example 2: When talking about M16A2 the MER is 550m point target and 800m area target. This change in MER was due to the change to 5.56 NATO vs .223. The twist rate was changed, the bullet is heavier and the muzzle velocity is IIRC 100 fps slower. These changes allowed reliable hits with desired effects at longer range.

Example 3: When talking about a sniper weapon system, the desired effects are the same. Reliable hits to the torso are capable at further range than with a standard battle rifle. These hits don't always have to be imediate kill shots, they just remove the enemy commander/machine gunner/mortarman from the fight. This is why tactical rifles must be capable of 1 MOA and not .2 MOA like the BR guys want. (even though I have seen tactical rifles shoot .2 MOA)


Example 4: When talking about a hunting rifle, the cartridge is capable of killing game at ranges further than most people feel comfortable shooting. This is not because they don't think they can hit it, it is because they don't want to chase a deer for 3 miles because they hit it in the gut rather than 2-3" behind the shoulder. (damn ethics always get in the way)

The MER varies based on shooter skill and desired effects. When a weapon system advertises a MER, they take into account the "average" ability of the person trained to operate it under "average conditions."

I hope this was helpful.