The WMD = guns line of thinking is one that gets trotted out with boring regularity by particularly stupid people. If confronted with this argument, you should smile, because pwnage is easy.
Your more idiotic breed of anti-gun person will simply try to draw a correlation between a firearm and a nuclear/biological/chemical weapon.
When that argument gets trotted out, the best thing to do is to point out the (blazingly obvious to anyone with more than three brain cells) difference between the two, and note that no serious governmental body would classify rifles as WMDs.
A slightly more clever breed of anti will try to use the argument to box you into a corner. On the one hand, if you claim that you believe that WMDs are covered by the 2nd Amendment, they'll simply point out that you're clearly a lunatic.
On the other hand, if you say that you don't believe that WMDs are protected under the 2nd Amendment, they'll try to use it as an "Ah-HA!" moment to claim that "see, you do believe in some form of weapons control" after which they'll try to tie that argument back around to why we should ban guns that they perceive as being somehow more dangerous than average.
There's no doubt that this particular argument is completely fatuous, but if you don't handle it with some level of care, you will end up looking stupid or being painted as a hypocrite.
When confronted with the second version of this argument, the best thing to do is to focus on a little thing I like to call "The Real World" and point out that there is no political movement in either direction for the right to possess WMDs, and as a result, you're much more interested in keeping the conversation focused on whether or not we should be throwing people in jail because they own black guns and/or standard capacity magazines.
This will make you look like someone who's honestly interested in taking part in a discussion, rather than engaging in hyperbolic theoretical discussions. It will also drive your anti-gun adversaries up the freaking wall, which can be a joy to behold.
I don't think we should let this terminology go unchallenged,but should expose it for the ridiculous hyperbole it is.
