NO! Why? It sets them up for agressiveness. Every conflict should not end with shooting someone (whether they are killed or not) and winning at all cost. Teach the child that art of compromise and compassion. It goes much further into adulthood as a positive life skill.
There's a blog post on the Hand in Hand website where a parent talks about a game she and her son made up around his desire to play with guns and I think games like that are a wonderful way to get to the underlying issues. Why does your child want to play with guns now? What is it that they are looking for in that play? Are there other ways you can meet that need? And what does happen if you playfully join in the gun play? What can you learn about your child and his world that way that will help you understand what he's really needing?