Tell said "expert" that if he wants fragmenting ammo with low chances of penetration to get himself some of the Hornady TAP or the Hornady Varmint Express ammo that makes use of the Vmax bullet. It will be frangible to the degree that it is downright explosive and have minimal chances of over penetration, if that's his concern.
What others mentioned about lead cores going downrange while leaving their jackets in the bore, it is true. I've seen a barrel from a 44Magnum that had shed it's jacket and completely plugged the bore during subsequent shots, the bullets were older jacketed open base/open tip bullets.
Also, AR15s can be very picky if they have flat spots on the front of the bullet. Some AR15s can't cycle the 52grn Speer hollow point or even lead tipped spire point rounds due to the rounds just barely catching on the corner of the feed ramps of the barrel extension. Best to use something with a nice pointy nose, much improved feeding.