Why would someone question another persons wants? This makes a lot more sense than a lot of crap my friends buy.
This would be one of the worst possible choices in the world in that capacity. Way too heavy to carry long distances or comfortably, a smaller back-up PERSON (wife) would not want to use it if i was incapacitated.
Very long engagement time. Self-defense shots are usually snap shots.
Very long 2d target engagement time. When three perps are coming after you, getting #2 in sights in less than the minimum 10 seconds it would take you to recover from this... (it would take me about 10 days) is very important.
So it shoots 1,000,000 yards. How do most urban, suburban dwellers justify shooting anyone not on their yard!
So it can penetrate homogeneous steel better than a proton torpedo. Today's bullets need to penetrate skin, or in the case of a totally armored human, a face or ankle.
So it blows them into microscopic biological specimens, the 30.06, M1 Carbine round and the .45 APC got us through WWII, and the 9mm x 19 was fine in German SMGs (although I do not like the round)
Very small total load-out ability, especially with a basic battle load of ammunition.
Why not bring a 280 MM AC?