Flag Code
Aquila B is correct, there is a Flag Code,and it is pretty specific about do's and don't of display of the flag. Title 4 U.S. Code. I have to take exception to his interpretation however. The code addresses the flag, patches, and what might be termed "images" of the flag.
Comments: Most folks are not aware that the forming apparel from the US flag is a violation of code, so a bikini, helmet, ball cap, etc is incorrect. So to the display of a flag image/patch on everyday clothing, or a sports uniform, excepting a flag patch on military, fire and police uniforms, and "members of a patriotic organization". The correct way to "wear" a flag (as a citizen) is a a lapel pin, on the left side of the body, nearest the heart.
One could make the argument that a holstered handgun is "wearing" and thus a grip displaying the image of the flag would be incorrect according to code. Additionally, when drawing and discharging a handgun, the "flag" would be obscured/covered, again a sign of disrespect. From an ethics standpoint, commercializing the flag as an image could be easily (and I believe correctly) argued as disrespectful. Most of what I have described above is largely, if not totally, commercialism.
Yes, it is all done. I am not aware of any entity that "enforces" the flag code, you will not be arrested or cited to court for wearing a flag T-shirt on the 4th of July, or at your gun club. But do consider that there is a right and wrong way to wear and display the flag and it's images, and much of what passes as acceptable is incorrect. As a symbol of a living country and the brave men people who have stood for it over many years, I strive to do it "right" and encourage others to do the same.
For those wishing to learn more about the Flag Code, the Sharpman Company has published a very readable book, with examples and commentary, titled "The Care and Display of the American Flag". My copy is copywrited 2004.