Big Jake.
With a 16" cone, and "not aiming", you will lose at least half of your projectiles.
On top of that, your projectiles are not large enough in diameter to reliably drop blood pressue in major organs.
Most of those major organs, especially for a frontal shot, are protected by bone and cartilege.
The recommended penetration is NOT 8", but 12".
The reason for the 12" penetration is to account for a number of factors, two of them being an arm, bone or thick clothing being in the way, and still getting sufficient penetration to reach vitals, or a very large fat/muscled person with a wide torso (fat or body builder types). In other words, the sort of people you would need a gun to defend yourself from in the first place.
I've seen a number of violent, large type who easily have more than 4" between skin and vitals in the torso, and if you add a thick jacket, you can pretty much forget doing more than pissing them off. Even a small gun like a .380 with ball ammo or hollowpoint would have a MUCH better chance than the .410/45 colt loaded with birdshot of doing enough damage to STOP such a person immediately. Even if you did manage to score lethal hits, you would be EXTREMELY unlikely to score immediately stopping hits. The guy would die, minutes or hours later, after he beat you over the head with whatever was handy. You would have to be luckier than James Bond at the roulette table to score a debilitating hit on the central nervous system with that load.
If you use that .45 colt with a birdshot load. Make sure you grind off the front sight blade so it hurts less. Might want to oil the outside of the barrel real good too.
Recap: Won't penetrate enough on big threats, especially with heavy clothes. If you manage to hit vitals with a few pellets, the wounds won't be large enough in diamater to cause rapid blood pressure drop. The guy may still be active enough to finish you.
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The formula for stopping a threat is simple. Start with a weapon that gives sufficient, PROVEN penetration to hit vitals, given a near-worst case situation (fat, muscle, winter clothing, bone). Then use accurate shot placement, and fire until the threat stops being a threat.
Any variance from this is likely to get you killed.
If you want to use birdshot, the barrel should be touching the perp's skin, at which point, you might as well use a solid slug anyway.
If you want to test your birdshot theory, find a convenient medium/large animal, piss it off, and shoot (make sure you have a game tag and hunting license). I would highly recommend the next five rounds be premium hunting ammo, since I sincerely wish you well.
