Easy, you just use the Joint tool, put a joint where you want the minifig to hold the item, and then after placing the Joint, go to the Joints tab, and highlight the Joint you just placed, then rename it to mountPoint, then for the shotting part, place a new Joint (after de-selecting the last one you put) and go to the Joints tab, highlight the one you just placed, then name that one muzzlePoint
And there ya have it