In game terms, I'd go with (Best-Craftsmanship) a Micro'd Long-Las with a Scope and Whisper-Bolt Discharger.
Is it the best possible Long-Las? Of course not. But it's ridiculously compact, and the Scope compensates for the reduced range. If you don't mind reloads, you could also slap a Hot-Shot Charger on it.
Some of that of course requires you to pull on cross-book material. Micro is from Rogue Trader, and the Hot-Shot Chargers are from Only War, I think.