Start with an Ares Thunderstruck Gauss Rifle
Add a major firing selection change and make it high velocity
Equip a Gas Vent 3 accessory and a gyromount.
You now have an assault cannon that deals a base damage of 9P with AP -half-4 that can go HV Full-auto for a base damage of 20. It's expensive as hell, though, and you'd never be able to actually get it, but it can kill anything.
I'm pretty sure that would take 8 modification slots, thereby breaking Jokers rule about it needing to be possible according to RAW. =P Other than that... sounds awesome.
My entry:
Start with an Stoner-Ares M107 HMG [7P dmg, -3 AP]
Gas Vent 3 (included)
Smartgun System (1 slot)
Electronic Firing (2 slots)
High Velocity (2 slots)
Improved Range Finder (1 slot)
Shockpad (0 slots)
Gyro Stabilization (0 slots)
Trigger Removal (0 slots)
In the end, you get a gun that fires 12 round long bursts, (+12 DV or -12 Defense pool), has 11 RC (AKA no recoil under normal circumstances), all range categories are reduced by 1, all perception checks to find where the gun is being fired from are reduced by 1 die, and you get the +2 dice from smartgun.
If combined with hawk eye (Positive Quality), you can shoot a target at 1200m with a full burst (dealing 19P base damage, -3 AP) and still be rolling (Agility + Automatics + 1) dice. And that's assuming standard ammo. Add AV rounds or hollow points to the mix, and it just goes downhill from there.