Small watched the two steal over $600 from the register of the Southwest Philadelphia bar.
Police say they also took $1,500 in cash from staff, not to mention an unknown amount from patrons.
"If you see a big gun, you get on the ground and give them what they want," Small said.
Police believe the gunman is behind three other robberies he conducted solo within the last month.
Police say the man hit first on September 7th at Good Times Bar in West Philadelphia. He can be heard on surveillance video demanding cash and cell phones. He orders the patrons to stay down and says "thank you, thank you" to them for his haul.
Police believe he also robbed Famous Bar and Restaurant in the early hours of September 30th and J.Z. Tavern just one day later, both in West Philadelphia.
"We're concerned, obviously, we have one in the earlier part of September and now we have three in last week so we're concerned that the frequency is increasing," Lt. John Walker of the Philadelphia Police Department said.
The gunman from last night's robbery is described as between 5'9 and 5'11 with a medium to stocky build wearing black clothing and sunglasses. The other is described as 5-2 to 5-4 with a thin build also wearing all black. They were both wearing black ski masks and gloves.
Police are asking anyone with information to call Southwest Detectives at 215-686-3183 or 215-686-3184.