If you have never heard of MilSim, it is simply an abbreviation of military simulation. In paintball and airsoft, a popular method of play is MilSim. On a more important level, the military often trains by conducting military simulations, often incorporating airsoft guns into their activities. While MilSim is sometimes played in a paintball scenario, it is most popular in airsoft, due to the realism of airsoft guns.
MilSim often involves the reenactment of wars or even individual battles of the past. For example, many MilSim games recreate popular World War II, Vietnam, and even Iraq war battles. To increase the realism of the simulations, players are encouraged to wear apparel and use airsoft gun models of the time. Some players even go as far as to use magazines that hold realistic ammunition capacities. Depending on where the simulation takes place, a game can last from a few hours to a few days. Longer games usually take place in a designated airsoft field and have considerable planning put into them, while shorter games can be relatively impulsive and spontaneous.
Due to the fact that MilSim is so popular, many airsoft guns are made as replicas of real guns. Airsoft companies know that airsoft players enjoy participating in MilSim games, and they have manufactured guns to suit that need. Some popular models used in MilSim games include M16s, AK-47s, and Steyr Augs.
A type of game similar to MilSim is law-enforcement simulation. In this kind of game, close quarter combat airsoft guns are more heavily used than in MilSim, and the clothing and tactical gear worn is also different. While MilSim players might utilize actual airsoft grenades, law-enforcement players may favor smoke grenades more. Although the differences may sound subtle, in practice gameplay is very different.
In summary, the key characteristics of MilSim include the employment of replica airsoft guns, time-specific apparel and tactical gear, longer-than-usual game times, and advanced preparation. Also, one team may assume the role of the "good guys", while the other team would play as the "bad guys" (in regular airsoft games, there is usually no designation of teams).