Humvees have windows. What does one see in a vehicle window? Driver's and passengers' heads, necks, and upper torsos. There have been attacks in both Icrap and Trashcanistan where grenades were thrown into the open windows. Truthfully, attacks such as these are hard to defend against. Where people have the will, they shall always find a way. Guerilla warfare (a term the U.S. military is reluctant to use because it implies a popular peoples' movement) is dependant upon hit-and-run attacks on soft military targets, of which the Humvee is one. Firing into a Humvee window is easily done. With a bolt-action rifle, even more so from a distance. The original Mujahadeen (who fought the Soviets) were using old British Lee-Enfields to engage Soviets personnel and light vehicles at dustances that allowed them to get away afterwards. Now the advantage of the higher-powered bolt actions becomes apparent, does it not? In Icrap, the guerillas take great risk to come up on a Humvee with an AK. There have been a few sniper attacks that may have been the work of 7.62x54 Dragunovs or even some bolt action sportng rifles due to the cicumstances behid them. But getting close enough with an AK requires a certain bravado. A bolt action military rifle, such as your surplus 8mm, your .303, your 7.62x54, your .30'06 can allow a skilled rifleman to hang back a little. This is why Afghans hung on to the Lee-Enfields they had even after liberating AKs. A rugged bolt action with good iron sights has advantages in a guerilla situation because usually, a stand-up fight with better armed opponents, opponents that outnumber you, and have better training and access to airpower means you will die. The semi-auto has advantages of its own, but a few well-placed shots here and there drains the resources of the opponent and hits them hard in selling the war to their own people back home. Living to fight for years as opposed to dying in a few weeks is what usually creates a win for a guerilla army.
"The guerilla must move among the people like a fish in the sea..."----Mao Tse-Tung
"Push forth a bayonet. If one hits flesh, push forward. If one hits steel, pull back for another day..."---V.I. Lenin