Title says it all really. I have no strong opinion either way but since it's likely to come up, we should have a meta question about it.
I think firearms in general would be off-topic for martial arts. Martial arts to me as a whole refers to the practice of physical movements (whether for sport or self-defense) with possible ties to mental and spiritual development (as is the case for Eastern systems).
If there are systems where firearms are part of the practice, and I really can't think of any, then the question should be on topic. If the question relates to disarming an opponent, like in Krav Maga, then it should be on-topic. Take note the question should be about the technique or practice and not about the firearm itself.
As a whole, using firearms themselves obviates the need for martial arts. The Wikipedia martial arts page states:
Europe's colonization of Asian countries also brought about a decline in local martial arts, especially with the introduction of firearms. This can clearly be seen in India after the full establishment of British Raj in the 19th century. Similar phenomena occurred in Southeast Asian colonies such as Malaysia, Indonesia, Vietnam and the Philippines.
and I would consider a historical topic like this (about martial arts's decline due to firearms, not about the history of firearms) to be on-topic. Why firearms are not part of martial arts is another question I would consider to be on-topic since there are multitudes of weapons in various systems, and I think knowing the historical development and use of those weapons is worthwhile knowledge.
If it's gun fu, stunt work, or wire work you're talking about, I would consider those to be off-topic too since they are more about movie production than martial arts themselves. There is a Firearms Stack Exchange as well, but I would be careful not to just shuffle all firearms questions there.
I don't personally care one way or another whether firearms are allowed as topics here.
However, to answer the other two people who ask if there are any martial arts that include attacking with firearms, there are.
Besides my friends who unofficially adapts firearms to their kata work, I know Systema and some branches of ninpo budo taijutsu explicitly teaches firearm attacks as part of their lineage. Systema gun work will use Systema body movements, much in the same way that the Chinese have archery kata based around variations of Horse Stance. In ninpo budo taijutsu gun attacks are variants of their knife attacks (using their opponents' weapons against them). (Caveat: this is all their public-domain demos; I have not seen any of their inner-door stuff or trained in their lineage).
My point is that in these two examples, the gun work naturally flow out of the principles taught within the lineage. They are not bolted on as an after-thought. They would also be the least likely to ask about techniques for using firearms within the martial arts since they would be deriving the techniques from the core principles of their art.
Actually, even discounting the use of traditional martial arts in adjunct to firearms, the use of firearms by themselves when trained follows a lot of the same principles and mentalities associated with traditional hand to hand techniques. As such, I'd think that they should be on-topic.
For examples see the wikipedia definitions of The Modern Stance of the Pistol (which includes the ubiquitous Weaver stance, and the modification of this introduced as the Chapman Stance), Point Shooting, and Center Axis Relock. These are very similar to stances as used in many more traditional arts, and understanding them can help even with the use of more traditional arts.
is there martial arts that use firearms? i know in judo there is at least 1 kata that has a couple gun defenses, but nothing with attacking with them.
perhaps some of the more military arts have questions relating to firearms.
i think questions like "in the art name technique where you do this and that with a firearm how/what/why do you do this and that should be considered on topic.
If it is questions about general firearm selection and use, then I would say it belongs on firearms.stackexchange.com.
However, if it relates to gun defenses, or incorporating any weapon (not just firearms) into martial arts, then that should be on topic.
For example, questions like "How do I turn the safety off on a Luger?" would be off topic. Questions like "How do I disarm someone when the gun is pointed at the back of my head?" would be on topic.