Tell him/her that many good things can hurt if not treated right. Mommy and daddy cook on the stove, but stoves can burn you. Guns are like that.
Let him/her touch it (remove the mystique) with gun in a totally safe condition, but make it clear he/she is not to do that w/o your permission - just like the stove. Also say that you'll teach more later - let the child set the pace.
Many children quite young have gotten involved in supervised shooting.
As your child progresses in life, keep up with the natural curiosity and increase the knowledge. The real danger comes with the lack of knowledge, not the abundance of it - still younger ones require small doses, fitting of their attention span.
When appropriate say that you never want to harm anyone, but that loving and caring means being willing to protect the younger ones too - it is what responsible/good daddies do..........and give him/her a great big hug for me.