You're right, it's not a main argument for any of these.
It's not just applied to drugs. The whole "back alley abortion" meme is based on this same argument.
I have no agenda here either since I feel the same about all three issues.
My point here is just about the inconsistency.
When it comes to that particular argument, I don't think being inconsistent is a big deal.
It's an argument from practicality. All arguments from practicality stand relative to arguments from ethics.
If there's an overwhelming ethical imperative for someone about one of these issues, then the practicality of it can become less important due to the relative importance of the ethical stance.
Inconsistency is only a huge deal when one is inconsistent in their ethics. Being inconsistent about practical stances is just showing an understanding of their different factors, or possibly in one's view of how ethically important they are. And there's not anything hypocritical about that.
I think, also, they kind of exist in different places on the spectrum of personal rights. Again, I'm pro for all three, but I think it's easier to argue against guns from the perspective of how they can be used. For example...
I'm pro-choice due to personal rights, and there is never any situation in which a woman could possibly abort for any reason other than her personal rights, because the ZEF always exists in her body.
I'm pro-legalization, and although dosing someone is possible, it's possible with prescription/legal drugs too, thus rendering it irrelevant. Dosing someone is not unique to illegal drugs, so I can argue it from a pure personal rights perspective.
That's harder to do with guns, where it's irrefutable that they can be used to victimize someone and that they are uniquely deadly. Now, I think there are plenty of ways to mitigate this point as much as reality allows, and I don't think it's an overwhelmingly difficult argument to overcome as long as one's pro-gun position is well-rounded and comprehensive. BUT, I am just pointing out that it is different from the other two for this reason. For me, because of that, arguing pro-gun is not only about personal rights, but also about social culture and demonstrated models, which the other two aren't as much.
Because of that difference, issues of practicality can be given different weight, depending on a person's position.