"Light" Forces in DS2

I've been trying to put together a hi-tech "light" force similar in
spirit
to the proposed American "Medium Armor" units, i.e. a lightly-armored
hi-tech force with all vehicles having a common LAV chassis.  Most of
the
vehicles are easily doable, and many of them are similar to John
Atkinson's Space Byzantine "Akritai" vehicles :)  But I've found it
impossible to implement the "gun LAV", i.e. the one with a 105 mm gun on
a
turret.  From the relative effectiveness of the 105mm against tanks, I'd
say it's definitely a size 4 gun, but there's no way a size 4 weapon can
be put on a turret mount in a size 2 vehicle. (which is what I've taken
LAVs to be)  Has anyone else dealt with this problem before?  Thanks.