A short game is for people who just want a short and simple game. This game length consists of four regular waves and a Patriarch wave. Mid-wave specimens will start to spawn on the 2nd wave. Fleshpounds and Scrakes will start to spawn on the 3rd wave.
A long game is the longest game length currently available in Killing Floor, and the most commonly played, consisting of ten regular waves and a Patriarch wave. Mid-wave specimens will start to spawn on the 4th wave, and Scrakes and Fleshpoundss will start to spawn on the 5th and 7th wave onwards respectively. The specimen count can increase to ridiculously high values - up to 250 per wave - and therefore is the hardest to survive due to the amount of ammo the squad has to use.
All map-based achievements require a Long game.
Players are allowed to choose how many waves there are before the final boss with their own custom-length. However, when playing with a custom length, perk progression and achievements are disabled; but, players may still use the perk levels they have earned.