I wear gun shirts all the time on flights (about 200,000+ a year) and I have not had a problem yet. Yes, I have offended people, but hey, there are some others out there wearing stuff that is offensive to me, and do I get to complain?? Most of the time I get positive comments, or "where can I get one". I just love sitting next to some anti for 5 hours wearing my "Diversity" shirt.. or that tye dye hippie that is stuck back 30 years with my "Chicken Foot" shirt.
On International flights I wear stuff that is strictly business casual, more to blend in than anything else. There are a lot of places I go where this stuff would not "fly"..
-Bidah