OK, so the general consensus is that the steel shot will harm the barrel... It's all just confusing because I was under the impression that the hevishot was even harder and denser than steel shot and the gun has had lots of that pumped through it with no visible damage (my dad refuses to shoot steel for waterfowl).
@nitestalker I got the Mossberg info from http://www.gunpartscorp.com/CrossReference.aspx
If you look it shows the Western Auto 310AB as the Mossberg 500AB.
I really appreciate all the feedback from you guys but still don't know what to do haha!