Just a thought about the Gauss:
Hypersonic weaponry over a very very short field.
Everything folks have talked about in terms of ballistics is absolutely correct for most conventional low muzzle speed weapons.
But (assuming no mystical tech here but were talking about) This would be akin to taking a good sniper rifle, and trying to hit a slow moving target at 25 yards.
I am quite positive that someone can look up the muzzle velocity of a Gauss, but I find it hard to believe conventional ballistics would move it more than a small distance over even its max range.
Now, since were talking about this rail-gun strapped to a moving vehicle hitting a moving vehicle things can change of course with all of those variables.
I would hazard a guess of saying, that we will have different 'leading' per weapon. In increasing leading order it would probably have to be something like:
1. Las (light speed)
2. PPC (Light bolt ??speed??)
3. Gauss (Hypersonic)
4. Ultra-AC (Lighter caliber higher speed, as noted by higher affective range)
5. Lbx/AC (Lighter caliber higher speed, as noted by higher affective range)
6. MG's (conventional ammo rules apply)
7. SRM (missiles are slow mkay?

)
Id assume the cones of fire would shrink in relation to speed of weapon/distance and be added to for the Lbx shotgun affect.
That is my 2c.
-D.o.a.