First it will not hurt the gun shooting shot down a rifled barrel but the lead will foul up the rifling and effect accuracy even with just one shot. As for him shooting a 12 gauge taht is up to him and what he can handle. I would let him shoot it a few times and see what he thinks. A 20 is plenty for deer out to 100 yards. Also you DO NOT need to shoot a 3" slug for deer. 2 3/4" rounds were used for years and people never had a problem with them. When eh dose get a gun get a combo with both barrels. They are not that mcuh more and are very useful. I have a Mossberg 835 combo and love it.