AER9 Laser for me I think as a "universal" use against all gun, sears flesh, melts metal. Cheaper to run than a Gauss, and the best of the common types of arms found lying around, if I raided a lab or military instillation I'd try to find a Gauss because of the sheer kinetic power it has.
The AER-9 Laser Rifle
For indiscriminant use against all things hostile or otherwise undesirable in their current living state.