It's in the game so it's canon. Some people have a hard time coming to grips with that, but that's the way it works.
That said, I do think NV had too many weapons which are almost identical to weapons in our universe, and I think it kinda knocked the immersion a bit of the whole Fallout universe.
They're fun to use, sure, but they just don't feel very Fallout-y.
How does having weapons that exist in our world ruin your immersion into the Fallout world? If anything that should help solidify the fact that the Fallout universe is, for all intensive purposes, an alternate plane of our own reality. Just because weapon development started on a new path of lasers, plasma, electricity, and whatever other wacky things we've seen in the Fallout series thus far. It doesn't mean that they would just start getting rid of every firearm from WWII and the conflicts preceding it. I'm sure that during the time frame of advancing energy weapon technology, the conventional firearms were still being toyed with, advanced, and brought up to speed with the rest of the warfare tech. Nothing, I mean nothing at all, beats the smell of gun powder and of course the lovely noises conventional firearms make. :goodjob:
The first 2 games have lots of weapons that would "break" Canon. But you never hear anyone complaining about that.
It seems Fallout 3 really made people insanely protective of "canon", eh?