[@Elgappa] raises some reasonable and excellent points and I pretty much agree with what he says. I'd just like to throw my two cents in the ring and talk about a specific sentence he said:

[quote=Elgappa]With a reasonable and fitting explanation you might even smuggle a laser gun into an rp[/quote]
Technically yes, that's true. However, it's not only the explanation that really has to carry this wacky concept. You also have to make sure that the inclusion of such an anachronistic (and potentially imbalanced, though it's less of a personal concern for me) object does not ruin the theme the story is going for. In other words, only do it when you:
1) Know you can afford to and know how to present it.
2) Know that it will strictly improve the story as opposed to damaging it, good presentation notwithstanding.
I essentially agree even with your point here, but felt it needed some slight nuancing.