Many gun tattoos feature the weapon by itself. It is not unusual to see two guns placed on either hip, for easy access, as with a holster. They are also frequently seen along the top of each shoulder blade, or on the lower stomach, so as to give the appearance of being lodged below a person’s belt.
Although it is quite common to see modern hand guns, shotguns, and even machine guns, it can be very unique to use a more antique equivalent.
For instance, a long barreled Colt revolver from the 1800's might make for an interesting choice. With this variety of gun, you could expand on your design by adding scroll work patterns to the barrel, gems and wood to the butt.
