The Mk. System is used in 2001 Brikwars. Mk.I = 1d10, Mk.II = 2d10, etc.
2001 rules have some, and I made some
to work with 2005 rules (the link is to fire in general, but the stats for a flame thrower are in there too).
When you target anything, you get a bonus for how big it is. The ground is (for Brikwars purposes anyways) infinitely large, so you'd get an infinite bonus to skill. Of course, your enemy gets to pick where on your target exactly the shot lands. Think of it like shooting a minifig with a rifle: you may hit, but whether the shot lands in the arm, leg, head, or torso is pretty much up to the target (damage is also exactly the same when it comes to what body part is hit, unless someone specifically wants their unit to have taken a headshot and get killed).
So basically, shooting at a larger patch of ground is easier to hit, but the shot can land anywhere in that patch.