This has been going around for a long time. It is in fact video of rock chucks, an often hunted varnint. There is no indication that any of the shots are of people. The video tape/DVD "Prairy Dogs Be Gone 2" from velocity films shows the 50 BMG bolt rifle used on large varmints. I have this video, but cannot attest that this footage came from this particular DVD, but it may have.
People in the position to fire on enemy combatants with small or large sniper rifles do not make video tapes for you tube showing off their exploits. People shooting varmints at long range with cool equipment do.
Any footage of US military snipers is very rare and never shows the target in a tactical situation. About the only way to see actual enemy eradication is the attack helicopter footage that is realeased. In these videos the enemy is mearly vaporized by 30mm cannons leaving a glowing heat signature of jihad goo. Search live leak for one called "dog escapes" it is one of the better ones.