The most effective wounds that will incapacitate an attacker are either to penetrate and destroy vital organs, CNS, or to disrupt enough tissue to cause skeletal or muscular failure. Bleed out takes too much time, "energy dump" effects are near minimal at pistol velocities, and temporary cavity volume is also minimal, and is useless unless it can destroy tissue. More or less, a pistol round has to punch through the boiler room, or damage enough tissue to make a continued attack physically impossible. One thing to consider in terminal ballistics of pistols is the volume of the permanent cavity and it's depth, both increase the chances of destroying organs, and a resulting incapacitation, this holds true in both 2 and 4 legged targets. 5.7X28 has a bullet .224"X.750 that does not expand or fragment, but yaws, so at 9.5" of penetration assuming it is completely sideways 1/2 of the path, the wound channel will be just over 1 cubic inch. A 9m that penetrates to 11 inches, and expands to .6 after 2"" will have a wound channel of 2.7ci. A 45 that penetrates to 13" and expands to .750 after 2" will have a wound volume of 5.1ci. Just for comparison a 22lr 40gr round nose at 13" of penetration will have .5ci of wound volume.
The capacity is indeed impressive, but if you concider maximum possible damage volume per mag not even concidering most defensive shootings involve less than 4 rounds fired. 7rds of 45acp=35.7ci, 15rds of 9mm=40.5ci, 20rds of 5.7X28=20.2ci
so with say a 50 or so grain boattail round, how much worse will it perform out of a smaller casing than if it were in a 5.56... i cant imagine that it will be that much worse especially considering the energy at impact and the speed of travel...
5.56 ball ammo fragments above 2700fps, and scatters shards of core and jacket at depths up to 14" deep randomly perforating and area upwards of 15ci, increasing the chance of rupturing the vitals. 5.7x28 simply does not employ this wounding mechanism or penetrate this deep.
There is no question a 5.7x28 can potentially stop an attacker, or kill, and the 20rd capacity is impressive, but the numbers, tests and ballistics hint at the likelyhood a COM shot has of doing what is required of a defensive firearm, and the size is somewhat ungainly for CC, ultimately you should carry what suits your ability, and what chambering you think provides adequate performance. I would rather be confident in my choices performance, than confident it would not overpenetrate, as the very real possibility of a miss to me kind of negates that concern for the most part, and most Bad folk tend to be quite a bit tougher than a couple sheets of drywall.
) 
