I don't own any T-shirt with a pro gun message. I'd rather not advertise to potential aggressors that I might be carrying a concealed weapon.
Rather, I dress in public so that if the day ever comes that I have to draw and fire my weapon, the aggressor will exclaim to the investigating officers:
"Honest to god! I had NO idea that balding fat guy had a gun! He didn't look like the type! He didn't even have an NRA sticker on his SUV!"