The following is only my opinion. It is based on some experience with our legal system as well as some "off the record" discussions with friends who are lawyers.
First, no matter what is or isn't in the law, what really matters is what you can get a jury to buy. Let's take a step back a minute and assume there were a different dangerous condition...
Suppose someone slips on ice in the parking lot. There's going to be a giant difference in the outcome of the case...usually....if, say, the ice has been there for a week with people slipping on it until finally someone got hurt....and a patch of ice that has just formed in the past hour due to a snowfall. The law (and juries) will go after the former...but give a lot of leeway to the latter. If there is an established, known, specific risk that they do nothing about...you're going to win your case against them. Just because something bad happens on their property that they had no way of knowing about and preventing, however, doesn't guarantee you anything. They might settle out of court if they view it as cheaper, or they may fight it (and win) on principle.
Same thing would *likely* apply to a firearms situation. You'd have to show a pattern of threat (the place regularly getting robbed, for example) with a lack of attempts to resolve it (no hired security, etc)...and even then, unless you can somehow show that you _would_ (not just "might have") been able to avoid an injury by being armed (good luck selling that in this state...) you're facing long odds on winning the case.
To see it put in other words, by attorneys....
http://www.domnitzlaw.com/Articles/Proving-Liability-in-Wisconsin-Slip-and-Fall-Cases.shtml
My opinion is that any place that wants to post should be using metal detectors and provide free storage.