Introduced in the US broadcast of Mobile Suit Gundam SEED, disco guns are to firearms what tea is to alcohol and the digital bikini is to nudity in anime broadcast in the US - and they were more akin to the former, sadly.
Basically, BS&P decided that the use of firearms in Gundam SEED would never pass the censors (especially considering that they would be used to threaten characters who were 16 years old), so they decided the best thing to do would be to tart the guns up a bit, to make them look like sci-fi weapons. The end result, on the other hand, looked like the 70's vomited over a Glock. Yes, disco guns were THAT ridiculous - they made the "tea" in Tenchi Muyo! look like a stroke of brilliance. (Yes, they were THAT bad.)
One can only imagine the amount of cognitive dissonance (or psychotropics) it took to even THINK these were a good idea...