What I need is a good-quality, reliable rifle that can be easily broken down to fit in a standard small suitcase, of a size that would pass as airline carry-on luggage without a second thought.
I've googled things like "collapsible rifle", "takedown rifle" and "take-down sniper rifle -airsoft", and spent entirely too much time browsing Sniper Central, IMFDB, and various manufacturer's websites, as well as the firearms tag here. Unfortunately, most of what I'm finding is, sorry, overkill for the job in question. I hesitate to even say I need a "sniper" rifle; I need sniper-level precision, but the range is well within that of an ordinary rifle.
Bolt action is fine; if they don't get the target with the first shot, they're not going to get a chance for a second.
The shooter is an experienced sniper. They've had time to get familiar with the area, and will have a decent if not wildly generous amount of time to set up.
The shot is from a fourth-floor window to the opposite side of the street -- probably less than a hundred feet total, if I've remembered my high-school geometry correctly. So not that challenging in terms of distance, although the angle's a bit tricky. But there will be several innocent bystanders, so precision is vital.
The rifle is being provided by an off-book government agency, so cost is not a major obstacle -- but if I'm going to use a several-thousand-dollar firearm, I'd better be able to justify it on a fictional expense report. I definitely don't want any rare guns or fancy collector's items; the rifle's being trashed when the job is done, and it would break my character's heart. ;-)
Bonus points if the rifle is of Russian manufacture, but it's not a necessity.