tl;dr version: To my knowledge, we've had three weapons-related incidents, but only one was actually a gun in the legal sense. Each got the individual a stern talking-to, but that's mostly been it. We have no policy prohibiting any sort of weapon, because if you hold it right, every single thing in the space is one.
The first incident was the repeated discharge of a Gauss gun without adequate regard for safety. In addition to the private discussion, there was a general reminder to all members, that the old rule "know your target AND what is beyond it" applies here too, and please make sure everyone in the space has plenty of notice, opportunity to take shelter, and opportunity to object, before any testing of energy weapons. The individual is no longer a member, because of a safety-related but not weapon-related incident some time later. (Our only ban!)
The second was the rupture of a poorly-constructed PVC pressure vessel (an oxymoron already), which was on its way to being a t-shirt cannon. The individual behind it was asked to get second-opinions on his "engineering" before assembling anything else containing substantial potential energy. (In retrospect, it's sort of surprising that none of his previous robots managed to fail spectacularly, but you can't really ban springs. Or giant rubber bands. Or giant clumps of small rubber bands. Or torsion bars. Or...) 
The third involved a member with a permit to legally carry a concealed firearm. He failed to lock it up before consuming alcohol, which is against the law. After consulting with numerous other members who carry concealed and follow the law when they do so, the individual was pulled aside for a private conversation, and the availability of lockers was reiterated. 
Safety-wise, the table-saw has hurt more people than every other dangerous thing in the space, ever, combined. And we're not banning saws, because they're useful!
-Nate-

Pete Prodoehl  wrote:
>                 Here's a quick question for US-based hacker/makerspaces...
> 
> Do you have a firearms ban at your space, and if so, what are some of 
> the reasons behind the decision?
> 
> thanks!
> 
> 
> Pete
> 
> _______________________________________________
> Discuss mailing list
> Discuss@lists.hackerspaces.org
> http://lists.hackerspaces.org/mailman/listinfo/discuss
>                 