The simplest way to go about it, in Poser 6, is to first place the figure in a gun-holding pose, then enable IK on the arms, then parent each hand to the gun. Doesn't seem to mess up the wrists quite as badly as in earlier versions, though it can still boggle the shoulder-joints if you move the gun too far. The problem with this approach is that the gun remains fixed in space, and can't move naturally with the figure. You have to keyframe its every motion. It's usually better, in my opinion, to instead parent the gun to the leading arm, enable IK on the off-arm, and then parent the off-hand to the gun. Then you can aim with the leading arm, and the off-hand will follow automatically.