First off, make sure the smg is empty. Pop the mag out and close the bolt. Look towards the back of the smg and you will see a slot cut in each side of the top cover. There is a clip that goes across and locks it into place. Push the clip towards the front of the smg and pop the top cover off. Then remove the clip from its slot in the reciever and you can pull the recoil spring assembly back and up out of the bolt. Next, slide the bolt to the rear and once far enoguh back you can simply lift it out. Next, move to the front of the smg and look at the barrel nut. On the right hand side is a tab next to the barrel nut. Push this tab in and you can unscrew the barrel nut. Once it is unscrewed you can carefully (so you don't scratch the barrel) slide the nut off and then remove the barrel. The firearm is now field stripped.
I do not think that one can change from a rifle to a pistol with the classification, so you will always be stuk with the 5 round mag rule.