two things: If your reticle dot is red, you can hit. if your reticle dot does not turn red, he is outside your max range. I would HOPE you knew this already.

Second, and more likely problem: Lag spurts can cause snipers to lose shots, just like Forge guns can misfire and not throw the round downrange (Yet still consumes the projectile). I, however have been happily murdering snipers in the redline and on towertops with my covenant sniper rifle with rather consistent headshots.

I believe number two might be your problem. Either that or the Sniper Rifle is suffering from the same "No shot" forge glitch that heavies deal with.

Bullet travel time sir, shoot where they will be, not where they are.

sniper rifles and Forge guns do not use hitscan, unlike the standard assault/combat/rail/Scram. You actually HAVE to lead the target a bit.

There's actually projectiles that travel, and has been since closed beta.

we do have the same glitch that other weapons have, where sometimes a bullet doesn't hit, it's not limited to snipers and forges either, i've also seen it on plcs and burst hmgs.

when your sniping the best fix is to change to a sidearm and shoot a round off, if that doesn't work try moving a little bit, still no joy then try changing suit at a supply depot, if it carries on then try re-logging.

also max sniper range is 600m currently.

So you don't redline snipe? If not my compliments. I usually expend effort to cut pansy snipers out of their redline hidey-holes.