im no gun smith but it sounds to me that the gun has some rifling or barrel problems.a miss placed rifling could make the bullet spiral to unknown areas such as u said if it was a swell in the barrel it would lodge the bullet or blow up the barrel. from wat i can tell in your post it sounds like there is some sort of a defect in the rifling in the barrell swaping and changing everything like the youngman did means that its none of thoose. the rifling might be fine but it might be that they accidentally put to few or to many rifled grooves in ur barrel. plus a stock and other accesories like that really dont have a large impact on accuracy. id have the barrell cheked by a good gunsmith, if they stil say its not that then have it boresighted and set into a gun vice at a range and put a box of rounds through it start at 25, 50,100. but id try shooting it out of a vice that would cut down the shaking big time and should make a difference if its not anything else.