It actually ruined the time setting for me. The game was supposedly set in the mid to late 1960s, but most of the hardware, not just the guns, was from the late 70s and early 80s. Some of it was even from the 90s. I mean they weren't off by a year or two. The most annoying thing about it is there is more than enough period-appropriate military hardware to populate their game with as many guns as they wanted, so I was kind of disappointed by just how not-at-all-60s the whole aesthetic was.
The supposed story there, though, is that Activision wanted Treyarch to make a MW2 clone, but told them to set in the 1960s to avoid a lawsuit. They got sued anyway and settled out of court for an undisclosed sum.