I didn't play MW2 online much, but my brother who is a CoD fanboy says they sort of nerfed the M16 in BOps. Like in MW2 it was great at any distance and it would bring dudes down really quickly. Now, though, it really sucks at close encounters, and doesn't bring down dudes nearly as quickly.
I guess they balanced it out which is a good thing imo. It seemed overpowered in MW2, and was sort of the 'go to' weapon of choice for most hardcore players, much like how the auto M16 was in Counter Strike for example.