This sounds like one of the 'victims' had a concealed handgun, but it's not entirely clear:
Victim Shoots Suspect during Alleged Attempted Robbery
A 27-year-old man was shot by his robbery victim early Sunday morning during an attempted robbery. Around 2:30 a.m. on June 10, four employees of the AMF Bowling Alley, located at 6228 North Kings Highway in the Penn Daw area, were closing up the business. They exited and were locking the doors when they were approached by two masked men with guns. The suspects forced the victims back inside the closed business and announced the robbery. One of the victims, a 22-year-old Alexandria-area man, grabbed his own weapon and shot one of the suspects. The second suspect fired rounds at the victim as the victim ran outside.
The injured suspect, Ronald Whaley, ran outside but was unable to flee due to his injuries. The second suspect fled in a black-colored sedan. The victim was not injured. Whaley was transported to Inova Fairfax Hospital and treated for his non life-threatening injuries. On Monday, June 11, detectives served warrants on Whaley at Inova Fairfax Hospital. Ronald Whaley, of 411 Lakeview Drive in Temple Hills, Maryland, was charged with attempted robbery, abduction, use of a firearm in the commission of a felony and attempted aggravated malicious wounding. Detectives are still looking for the second suspect, described as black and driving a black-colored vehicle.
Anyone with information is asked to call Crime Solvers at 1-866-411-TIPS/8477 or Fairfax County Police at 703-691-2131.