According to this gentleman, the counter-snipers were probably using heavily modified Remington Model 700 bolt action rifles chambered in .300 Winchester Magnum:
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“Trouble rather the tiger in his lair than the sage among his books. For to you kingdoms and their armies are things mighty and enduring, but to him they are but toys of the moment, to be overturned with the flick of a finger.”
― Gordon R. Dickson, Tactics of Mistake
― Gordon R. Dickson, Tactics of Mistake