'Don't react and try to distract' is my imperfect way of going about this. I have a friend who managed to ensure her daugher didn't even know the word 'gun' until she went to school. I was so in awe of that accomplishment.
The first time round, when my oldest was an only child, it was easy to avoid having guns in the house. But once he hit school, he began to build up an arsenal or guns and swords. It wasn't that I suddenly changed tack, but toy guns are so very easy for school children to come across.
And my son asked 'why can I have a huge water pistol when I can't have a toy gun? Very fine line between the two. Have you seen the size of water blasters!!
My son bought guns with pocket money when my back was turned at the school fete, won them at the fair, got them in swaps with friends - the ways of acquiring them were many and devious. He was never gun mad, but he sensed my antipathy towards them, which of course made them more attractive.
When I had son number two, five years later, he was born into a house equipped with toy guns, not to mention huge water blasters, light sabres and swords. At just under two and a half, he isn't yet playing with them of his own accord. He's far more interested in cars at the moment, but sometimes he brandishes one in a playfight with my older son.
Because I now know the inevitability of him getting toy guns - not to mention making them out of twigs and bricks, I am much more tolerant of them. I just try to keep my reaction minimal, never join in gun play, and never ever pretend I've been shot dead. I don't want him to equate guns with power, and I feel very, very uncomfortable with the idea of him shooting mummy. As soon as he loses interest in the gun, I put it out of reach, and try to make sure that no guns are mixed in with his toys.
One thing to consider with gun play: much better to play at shooting, than to actually hit or bite another, IMO.
Also, I wonder if the surge in testosterone levels that boys have around the age of four, according to Steve Biddulph in 'Raising Boys' has anything to do with their interest in guns. I'd bet my bottom dollar it is! So is gun play a relatively harmless expression of their male side, however much we as peace-loving, PC mothers hate to see the little monsters ratting around?