The Marksmanship (MKS) attribute is a character's ability to hit a target.
If the character has 0 or 1 accuracy, they only ever hit on a 6 shots, or 9/12 when using 1in9/1in12 weapons.
If their accuracy is 2-5, they use base accuracy.
Characters with 6-10 accuracy can roll two dice, and select one.
Characters with 11-14 accuracy can roll three dice and select one.
Characters with 15 accuracy roll two dice and only ever miss on a total of 2 or 3 (that is a 1 and a 1 or a 1 and a 2).