I admit it. I grew up playing "guns" with my friends. And, honestly, I loved it! We would run around in our neighborhoods for hours having epic battles of cops and robbers. We each had our favorite toy gun. Mine was the mini-pistol that could be concealed in my tube sock. That baby was perfect for any prison escape I attempted (prison was behind the air conditioner - I often played the robber role!).
This video brought me back. Here we see a playful dad waking up his son with a squirt gun, singing the theme to Doom. Everything is done with total fun in mind. The kid is laughing and giggling throughout!
The topic of toy guns is a controversial one in parenting, especially when it comes to boys - who traditionally show the most interest. My slant is keep things playful. Remember just like in roughhousing, toy guns can go too far (i.e. if they are used to hit one another or if they are facilitating aggressive or violent behavior instead of playful fun). Most often kids know just how far to take their battles and it's rare that adults need to step in. I also don't know of any data that shows a casual relationship between toy guns and violence later in life.
Overall, I think if your kid wants to have a good game of cops and robbers or a classic squirt gun battle, my two cents would be let it ride.
Anthony DeBenedet, M.D. co-author of Quirk's The Art of Roughhousing: Good Old-Fashioned Horseplay and Why Every Kid Needs It.