I never, under any circumstances, use the terms interchangeably. However, there are plenty of people who have forgotten more about guns and shooting than I'll ever know who call magazines "clips". They're still wrong, at least technically, but I'm not going to correct them. Nor would I ever correct a fellow THR member on this topic. I'll gently correct non-shooters if the situation is appropriate. In the heat of an impassioned argument is not an appropriate situation.
If the only only argument that you can bring to bear in discussion with an anti critical of large-capacity magazines is that they're not called "clips", it's time to develop some better debate skills. On the other hand, if a friend refers to a magazine as a clip, I might mention that technically it's called a magazine, but that the distinction between the definitions of the two terms might seem somewhat obscure, and then return to the subject at hand. If they are interested in knowing the difference, great. If not, no big deal. It's a real technical distinction but it lacks much political or legal significance, and isn't worth pursuing too far in many cases.
On the other hand, the distinctions between, say, semi and fully automatic, or between assault weapons (an obtuse and largely meaningless legal fiction that disappeared, at least at the federal level, in 2004) and assault rifles are quite important, and I'll politely but firmly correct anyone who thinks they are synonymous.
