To quote from The Future of Play Theory: A Multidisciplinary Inquiry into the Contributions of Brian Sutton-Smith:
Findings from studies of war toys are diverse, if sparse. War toys
have been found to
- enhance aggression (Sanson and Di Muccio, 1993; Turner and Goldsmith, 1976; Watson and Peng, 1992)
- reduce aggression (Bonte and Musgrove, 1943; Gribbin, 1979)
- have no bearing on aggression (Sutton-Smith, Gerstmyer, and Meckley, 1988; Wegener-Spöhring, 1989)
Unfortunately, I wasn't able to find any subscription-free copies of those studies. You can find most of them here if you want to further your research.
My personal opinion is that the research is inconclusive due to a correlation/causation problem. Aggressive kids prefer aggressive toys, and play more aggressively with them than with other toys. It's very difficult to separate out whether the toys caused the aggression or vice versa. However, I have personal experience that strongly suggests the effect is temporary.
We had a two year-old foster son who came to us with toy guns in his personal belongings. I can't go into details about his background, but suffice it to say he had very good reasons for aggressive behavior that didn't involve toys. Although we don't buy toy guns for our own children (for reasons I'll detail later), possessions are very important to foster children so we let him keep them at first. However, we noticed that he hit other children with his guns far more than with any other toy, or bare-handed, so we took the guns away. After that, teaching him not to hit was much easier. In a month or two he had almost completely stopped.
All our other children have either ignored toy guns altogether, or consistently played with them appropriately, without it escalating into actual violence. Our one child where it mattered the effect wore off quickly, even being predisposed to violence. Bottom line is you are very unlikely to do permanent damage by exposing your children to toy guns in a trial period, to see how your individual child reacts, perhaps on a play date.
However, a reason you might not have considered is real gun safety. Even if you don't own guns, your children may encounter them at a friend or relative's house, or even discarded on the street. Children 6 and younger can't distinguish a toy gun from a real one, and for a few years after that, they can't distinguish a real gun without picking it up to gauge the weight. Even then, some real guns are extremely lightweight.
This is why a lot of parents teach their children to treat toy guns and real guns the same: stop, don't touch, leave the area, and tell an adult. It's not that it's impossible to teach gun safety with toy guns readily available, it just muddies the waters a lot.