If we are being pedantic, then the issue becomes fairly fuzzed when you consider guns that have you insert the clip, not just the ammunition, into a fixed magazine. The M1 Garand for example.

I don't know of any examples of guns with this sort of setup that would get anywhere near 15 rounds, but I suspect that the M1 specifically has something to do with the common misuse of terminology. It may also be related to some of the loonier misconceptions about guns, such as the idea that "clips" (actually magazines) are disposable (and therefore somehow also not reusable) (example of this misconception: http://en.wikipedia.org/wiki/Diana_DeGette#Gun_control).