OK, here's another "vest test" photo from the same occasion. This time the shooter is a uniformed officer, and he is using a weapon that appears to be of different make from the first shooter's gun. Looks like a round butt.
Vest Test: 1923 | Shorpy Historic Photo Archive
EDIT: Adding this caption: September 13, 1923. Washington, D.C. "W.H. Murphy of the Protective Garment Corp. of New York stood less than ten feet from [Frederick County, Md.] Deputy Sheriff Charles W. Smith in police headquarters Wednesday and let the deputy fire a .38 caliber revolver straight at his chest. When the bullet hit, Murphy never batted an eye. Inventors ot the bulletproof vest, which weighs about 11 pounds, have put [it on] the market for the protection of police and other officers in emergency cases. The bullet which Deputy Smith fired into the vest Wednesday was presented to him for a souvenir."