Remembering Einstein's formula of E equals M times C-squared, we have to admit that the smaller faster bullet can deliver an equal amount of energy as the bigger slower bullet. In fact, if engineered precisely, they could deliver an exactly identical amount of energy. But is it all about energy? We all already know it's not. As we said earlier, when stopping the aggressive action of the opponent, the goal - short of a central nervous system (CNS) hit - must be to cause sufficient damage to create a quick drop in their blood volume so that they drop.
Simple logic - with a focus on simple - would dictate that a bigger bullet makes a bigger hole and therefore does more damage. The challenge with that simple logic is that terminal ballistics have proven less predictable than we originally thought they would be. Bullets traveling faster than 1,000 feet person second do some strange and unexpected things upon impacting flesh. Hollow point, jacketed bullets sometimes don't expand as they are designed to do; other fragment for no observable reason. In other words, quite often our high-tech science does little more than allow us to take a good guess at what the bullets will do. In field terms we call this a "WAG" or Wild Ass Guess. In more specific and technical terms we call it a "SWAG" or scientific wild ass guess.
Past history shows us that people can die from being shot with a .22lr bullet. Countless hundreds of thousands have been killed by .223 / 5.56mm bullets. Velocity certainly plays a part along with multiple round delivery. There was a time when we had to separate the arguments between rifle calibers and handgun calibers, but with some effective rifle cartridges now packaged in handguns (i.e. the FN 5.7mm) the lines get more blurry than ever.
As we said earlier, the debate has long primarily been between 9mm (or equivalent such as .38) caliber weapons and .45ACP weapons. In common handguns today, if we're arguing capacity then the argument is small. For instance the Glock Model 17 holds 18 rounds of 9mm (17 in the magazine plus one in the chamber). The Glock 21 holds 14 rounds of .45ACP (13 +1). Is the debate really going to rage over FOUR rounds of capacity? If you are a fan of the .45ACP but want a higher capacity and so you switch to the 9mm to gain FOUR rounds, don't you think you should probably re-assess your shooting skills?
For decades the U.S. military found eight rounds (7+1) of .45ACP in a Government Model 1911 pistol sufficient. For most of those decades law enforcement found 6 rounds of .38 in a revolver sufficient. With two magazines for backup in the 1911 you had a total of 22 rounds of .45ACP. With two speedloaders for your .38 you had a total of 18 rounds. Certainly there were instances of officers and soldiers feeling under-gunned or citing examples of how those weapons and/or cartridges failed. There are also hundreds of stories about how well those weapons and calibers performed. 1985 changed it all. The debate wasn't settled but it was certainly quieted when the U.S. Military adopted the Beretta M9 9mm pistol with its 15+1 capacity. Adopted by the military and picked up by law enforcement agencies across the nation, the Beretta M9 or the civilian variant Beretta 92F seemed to silence the 9mm vs .45ACP debate. Maybe...
But there were still plenty of people out there who weren't confident that shooting someone with a 9mm round would provide sufficient immediate (or as close as possible) incapacitation. Other options were sought. That same year the .40S&W was born. Evolving out of the 10mm as the load was developed and changed in accordance with FBI requirements, the .40S&W cartridge ended up the same overall length as a typical 9mm Parabellum round. Since .40 and 10mm equal, the difference in diameter was only 1mm. Could it really make a difference? Apparently so. People and law enforcement agencies flocked to the .40 in untold numbers. In some pistols the change from a 9mm to a .40S&W caliber design didn't mean huge scrifices in capacity. Again, the Glock 17 9mm with its 18 round total capacity was compared to the Glock 22 .40S&W with its 16 round total capacity. Would those two rounds really matter? Many said no.
Then came compact and sub-compact models of .40S&W pistols. My latest venture into this arena is the Beretta 96F Centurion with its 11 round total capacity. The shorter slide, barrel and frame as compared to the Beretta 96F make the 96F Centurion easier to carry and conceal. However, since it's a .40 caliber weapon I hardly feel like I'm carrying a "mouse gun".