Instability in a short barrel is not a given. All things equal, the bullet exiting the 2" barrel is as stable as if it came from an 8" barrel. Terminal stability is a different issue and is based mostly on bullet construction. The main reason that the ammo companies tried to load heavy for caliber bullets in the old days is that they didn't have the jacket metallurgy technology that we do today. Most of the ballistic science in those day was subjective at best. For example, the tests to find a replacement for the 38 Colt military round involved shooting cows at the Chicago stock yards and observing how long it took for them to die and how much discomfort the cow exhibited. Sometimes they got it right, like with the 45acp, but most of the time it turned into junk science. Heavy for caliber bullets for self defense fell out of favor because of the advancement of the jacketed hollow point design. The bullet companies have been able to design bullets that expand over a huge range of velocities and impact mediums. This is what drives defensive ammo design now. But the primary idea that we must remember is that the only constant in stopping dynamics is shot placement. Shot placement trumps everything in factoring stopping power.