The .50 caliber round leaves the muzzle at 800 m/s. Quick and dirty calculation comes up with a 4.5-second travel time over 3500 m.
Your quick and dirty calculation assumes an 800 Meter/second velocity over the entire distance which is ballistically impossible. The projectile begins slowing immediately upon exiting the muzzle of the weapon or perhaps a bit sooner. The actual ballistics for the .50 Browning Machine Gun cartridge are:
- Muzzle Velocity : 853 Meters/second Time of flight 0.0000000
- 250 Meters: 781.69 Meters/second Time of flight 0.306105049
- 500 Meters: 713.51 Meters/second Time of flight 0.640873963
- 750 Meters: 648.8 Meters/second Time of flight 1.008348187
- 1 KM: 587.53 Meters/second Time of flight 1.413359571
- 1.25 KM: 530.11 Meters/second Time of flight 1.861528594
- 1.5 KM: 477.07 Meters/second Time of flight 2.359082422
which is as far as my ballistics table goes. However the reports of the sniper's shot indicated a 10 second flight time, which seems quite reasonable. Additionally ou have to take the bullets trajectory into account as it will travel significantly further through the air than the actual range over the ground. FWIW given the rifle is sighted in at 500 Meters at the 1500 meter range the bullet would strike at -665.91 inches or - 16.91 Meters.
I haven't attempted to run out the numbers, but at 3000 Meters the projectile would definitely be sub-sonic likely be moving at only 150 to 200 meters per second (perhaps close to terminal velocity) but because of the weight of the .50 projectile it would still hit the target (if it hit the target) with enough energy to do significant, potentially fatal, damage to an unarmored human body.