Actually, it expanded enough to make it a little wider than a full wadcutter, and it penetrated very well. The same basic load from other companies has a very good street rep. The FBI went to a jacketed bullet mainly to get better penetration on cars.
I dislike calling the lead form the Chicago load because that city has such nasty gun laws and odious politicians. Let's call it the Dallas or Miami-Dade or FBI load.
A gun editor who has shot some animals says that the expanded bullets most nearly resemble what those bullets do in water tests. BTW, he's shot alligators effectively with 125 grain .357's in a snub M-66 or M-19, I forget which.
Back about the time that Dallas adopted the 158 grain lead HP, I talked with Dr. Vincent DiMaio, who encouraged adoption after extensive lab tests. I think he used gelatin, but said that the round deposited well over 200 ft. lbs. of energy in the target medium. The old lead RN deposited just 84 ft. lbs.! No wonder it was a poor stopper!