The reason for the name is somewhat obvious; competitors use three different firearms — a modern sporting rifle (MSR), that is, a rifle built on an AR-platform; a pistol; and a shotgun. Matches generally involve courses where the shooter must move through different stages and engage targets in a variety of different positions. Each stage will generally require the use of different firearms and require the shooter to transition between them.
The targets might include clay pigeons, cardboard silhouettes, steel targets of varying sizes and anything else the match organizer designates as a target. Distances of the targets might vary from 1 yard to 500 yards or anything in-between. The shooter who hits the most targets — and avoids certain “no shoot” ones — in the least amount of time is the winner.
Although local ranges might opt to alter the rules somewhat, most abide closely by those set up by the International Multi-Gun Association (IMGA)