Jeff's flat full of it, on most occasions.
When he publish Giant Gunsite Gossip (vol 1) he invited criticism of it, so I sent him 170 pages, using nothing but his OWN words, where he'd said the exact opposite of something that he said in GGG, at one time or another (and never has recanted a word)).

For instance, in his COOPER ON HANDGUNS, he said that" a .20 gr .22 spitzer, at 2200 fps would be able to both penetrate any practicible armor, and deliver adequate stopping power, by means of hydrostatic shock."
Just leave out the armor issue, and take a look at the 223, which Jeff is so insistent about being "feeble". Well, start a 40 gr spitzer at 3600 fps, as can be done with the 223, and it will still have 2200 fps a LONG ways out there. Far beyond the range at which 99+% of INTENTIONAL, non-accidental hits occur in combat.
A 24" barreled HBAR, with 1" of flashhider. is JUST as short as is a 22" barreled M1A, with its 3" flashhider, and that 24" 223 will get a 75 gr Hornady, mag-friendly softpoint to 2800 fps, and it will still have 2200 fps left at 300 yds, and still expand in flesh at 500 yds(where it will still have 1800 fps, and 500 ft lbs. 40 grs at 2200 fps only has 400 ft lbs, and it's got 50% more momentum out there, than a 40 gr, 2200 fps slug has at the muzzle. Jeff's big on momentum, too.
When Jeff proposed making the 223 suffer the "minor caliber" scoring handicap, for IPSC rifle matches. I asked him why the .230 gr .45, at 800 fps (184 momentum-factor) was so superior to the 223's 70 grs at 3000 fps (210 momentum-factor). He just glared at me.

Where would you rather have a failure to stop, at 300m, where the guy can't hit you anyway, once he's suffered a ."mere" .22 hole in his chest) or at axe swinging ranges, hmm? Either the 223 has PLENTY of stopping power, or the .45 aint jack crap. choose.