My son is getting into trap/skeet/5 stand. He's shot some and is really liking it so I want to get him better equipment. He is 11 and is about 110 lbs. I've had some input on the 303 and 391 due to their dependability. An A400 is a bit too bulky for him. What I'm after is understanding the differences between the 303 vs the 391. Are any parts interchangeable? Also what is your general opinion of these 2 guns. Thank you in advance.