As a pointer: Given the number of nations that use alternative weapons/caliber an so forth, is it practical to pick up the gun from a dead enemy and use it in combat?
Or is NATO the best common denominator?
What exactly do you mean? Do you mean someone looking for and picking up a weapon and replacing their own? If so, for how long; permanently or temporarily? Or do you mean a weapon jamming and someone grabs the nearest weapon in a life or death situation, which happens to be an enemy weapon?
And who is the scavenger that you had in mind? Regular army personnel? Special Ops? Private military contractors?
Sure, in 1967 and 1973 Israel captured vast quantities of Soviet weaponry ranging from small arms to main battle tanks and even fighter aircraft. Much of this equipment was integrated into the Israeli arsenal, though some weapon systems were found inferior to their western counterparts and required substantial upgrading. A lot of it ended up exported to 3rd world countries that didn't mind buying obsolete tanks. Israeli engineers got so good at upgrading Soviet equipment that even today they contract out to ex-Soviet bloc states to improve their armor.