Of course the problem with a sign like that is that if anyone ever does get shot on the property, the jury will have a field day with it.
I can almost hear the prosecuting attorney's argument now...
While these signs might amuse gun-owning landowners, they probably fuel the gun-control crowd, and are therefore counterproductive in the long run.
On a positive note, I see the sign features an assult rifle and not something like a (broken, recalled, no fix available) Caracal C pistol.
(That might actually have encouraged trespassing?) So, kudos there.