Somehow I have a feeling that the GM has not exhausted nearly half the options possible to curb such behaviour. As has been pointed out, the act of simply getting into Point Blank range is and should be a feat all by itself. Sure, when combat has not yet started and you can surprise the enemy, then getting close might not be that difficult. That's kind of the point of being an Assassin. However, this only accounts for one kill, and if there were multiple enemies, the Assassin is now dangerously close to them.
Some more advice off the top of my head:
Enforce ammo and gear weight restrictions. Several hundred bullets are going to weigh "a bit" and, in addition to possibly being a cause for Encumbrance, may well negatively affect Sneak rolls. Magnus Grendel already mentioned this but it deserves to be said twice.
Keep in mind that ammo =/= ammo. There's no reason not to give enemy NPCs auto weapons as well, even if the Assassin will try to loot their ammo. This isn't the army, which means a cultist's or a ganger's armaments will be heavily personalised. Ultimately, your Assassin will have a hard time trying to fit the 9x40 mm slugs of Johnny Cawdor's customised AX-22 Imperator Longrifle or the caseless 6x35mm rounds of Lady Z's prized Ebony PL-10 into the magazine of their Alcher Mark II Justicar Autorifle that only takes 8x50mm. This is "a galaxy of guns", and projectile ammunition being non-standardised is one of the lasgun's most important advantages.
Otherwise, next thing you know it your Assassin may well start trying to force pistol or revolver rounds into their gun.
And for the record, I don't approve of the newer games' changes to the Autofire rules - the idea that even the very first bullet out of the barrel has a 10% reduced chance compared to a single shot just doesn't sit well with my sense of realism. The argument against autofire should not be a reduced chance to hit anything, but a reduced chance to have all bullets of your salvo hit compared to firing them individually, thus leading to ammo waste and requiring a higher use of the Reload action. DH's standard rules achieve this well enough (though I have an entirely different idea for alternative approaches .. but that's probably for another thread).
Edited by Lynata, 12 February 2014 - 02:30 PM.