Instead of the weapon being treated as SL2, how about a skill bonus to targeting a mini-figs head, and treating the target as SL1/4 with armor 2 (or countering armor bonus from helmets and going with 4).
The normal roll needed to hit a figures head is 5+, this could be reduced to 4+ with a +1 bonus to skill and any attack roll over a critical failure would result in a cloud of gory plastic innards.