My search strategy is turning up mostly Assassin's Creed guides and articles from way back when. I just want to look over any guns and weapons in general that an assassin with government resources might use today.
Probably some kind of long-distance sniper rifle. Have you tried "contract killer" instead of "assassin"?
Search for "sniper rifles" if you want long-distance stuff. Other successful state-sponsored assassinations have been carried out with polonium (radiation poisoning), ricin (poisoning), and in one case, the Israelis successfully placed an explosive-loaded cell phone with a (IIRC) Hezbollah leader, then detonated it while he was talking to someone. For most non-governmental assassinations, however, success has been achieved with the classic "pistols at short range" method. Archduke Ferdinand, John Lennon, Malcolm X, Lee Harvey Oswald (as victim), Abraham Lincoln, Mohandas Gandhi, Indira Gandhi, and Robert F. Kennedy were all killed this way. The downside is that the assassin is almost certain to be captured and/or killed, but statistically, it's a pretty successful strategy.
Try looking into the load out for a standard CIA or MI6 operative. Or maybe try looking up what Agent 47 from the Hitman VG series uses. But your best bet is someone who probably uses a revolver in a small calliber, or someone ex-special forces or something of the like. Though your best bet is a hell of a marksman with a long range sniper rifle.
A sharp icicle. Evidence melts. Now, seriously, I always thought Jason Bourne and the new James Bond movies had some pretty cool -- and usually realistic -- weapons, equipment, and tactics in them. Kingsmen is another great film with awesome gadgets and weapons in it, but I'm not sure how realistic your approach is.
I agree with @Iain Aschendale about poisons. With the ricin, it was done with an umbrella in a crowd, easy to slip away. Radiation poisoning also has a buffer in symptoms being shown. Cyanide is also hard to find in toxicological reports and will be untraceable after a couple of days. If the assassin wanted to kill multiple people (or could be sure to isolate one), they could use carbon monoxide as it's pretty much traceless too.
I'd generally agree with those suggesting sniper rifles. Pistols would be another option. Explosives also have a variety of possible uses. Place them on a vehicle, or in a piece of electronics, or even inside a suicide bomber, if your assassin is particularly dedicated and happy to martyr him/herself. The CIA once plotted to kill Fidel Castro with an exploding clam while he swam in the sea. Poison is possible, but you have to get pretty close to deliver it. I believe Georgi Markov's assassination via modified umbrella (to inject a poison pellet) has already been referenced. Then we get into weirder stuff. Like the assassination of a Russian dissident with polonium, or the plot to lace Castro's cigars with deadly bacteria.
That's very useful for me, everyone. The scene takes place in the near future, maybe ten to twenty years, so I can also invent some weapons. I do need realism, but I can slip a degree or two away from that. It's for one scene out of the entire book and I'm just looking to illustrate a few pseudo-government sponsored killers. Do you think I can get away with standard-issue military gear? Or would assassins need something more specific? And I gotta say, the exploding clam raised my eyebrows.
Suicide vest ...and one to ponder was only last week - the first reported ISIS use of a drone in combat. Two Kurdish guys killed, two French military injured. A while back I thought/wrote, I think - about this dystopia of anarchist drones, fat insects buzzing through our bathroom windows. The controller - long deceased, but devices still fly. And we live in bunkers, or maybe ride around on the back of our own security drones, wielding shotguns, blast these pests from the suburbs, something something... [It's been written already, reckon.] There's so much information about guns on the web? But I understand your frustration when a term [assassin] gets hijacked by pop brats - [sons of] Anarchy springs to mind. Though can't recall why I would google 'Anarchy?' What an idiot [dweeb].
If you're going with sniping, pretty standard issue military gear is fine. My experience is 20+ years out of date, but Marines are trained to shoot torso-sized targets from 500 yards with an M-16 (M-16A2 at that time) without a scope, so a lot of what civilians think of as "sniping" is just standard military training. Among the rifles that officially-designated snipers use are things that basically class as deer rifles, although usually with a lot of cool synthetic extras. Finally, the Barrett Model 82 .50 cal rifle can be bought legally in most US states (not California), and in the hands of someone properly trained is probably good out to a mile or so without raising any eyebrows.
The ultimate assassins weapon of all time is...(Pause for epic dramatic reveal.) Orbital Bombardment Lock on to their phones GPS and press the big red button. And poof they disappear.
Oh yeah a billionaire going out of his/her way to kill someone while attracting attention. ROFL! Detective: The cause of death was by a meteors!
