At that range it is pretty much a blind shot. You need someone to hold the enemy and then go off into the mountains. Aim at your teammate and have him guide you onto the target. It's a shame that this is the only way to get a 1km+ shot but the rendering distance is just too low for enemies and structures in game.
My best so far is 966 m using an un-suppressed MSR with T5Xi scope, long barrel and 10 round clip. THE ONLY WAY (that I have found) to get shots over 700m is to spot your target with a drone while you are 200-300m away, then scope in on your target with your sniper rifle and walk backwards WHILE SCOPED (the whole time) in on your target. At the 900 m + Distance you are lucky to see a floating gun and the tango symbol /type (sniper, lieutenant etc.) The HTI is an awesome weapon vs choppers and cars but to get the longest kill MSR is where you get money shots.
Yeah but MSR model is kinda oversized and looks a bit silly, clipping into the character and stuff. I stick with L115 for this reason, and that gun is no slouch either - 1413m with the usual shenanigans of a 'helper' holding the target and 870m 'real' kill. And there is no enemy MSR can kill that L115 can't, so...