The gang shot one of the boys three times with the weapon as they targeted the crane business Midland Safe Load Indicators Ltd, on Watling Street, in Brownhills.
A Ford Transit van, a magnetic drill, heavy-duty batteries, brass fittings and a pump jack were all taken from the site, which is also used to send shipments to Gambia on the weekends, around 4am on Monday morning.
Midland Safe Load Indicators Ltd manager Caroline Degville said she was left 'traumatised' for the teenagers and believes the site was specifically targeted.
The 50-year-old, who has managed the business for 10 years, said: "It is a crane business which we run Monday to Friday and I also run an export business on a weekend.
"We were working on a container yesterday and it was late when we finished. The lads stay overnight in the caravan and they are picked up in the morning.
"They were staying and around 3.30am this morning they heard the workshop roller shutters go up.
"It has got to be somebody that knows that we are here. I think they were targeting something in particular.
"I'm more fearful and upset for the boys. The gang, of around 10 to 15, threatened them and they had machetes. They heard them in the caravan and threatened to kill them.
"I am traumatised for the boys. The one has just been released from hospital and they removed a metal bearing, shot from the gun, from his head, just above his eye, and his hand – that is how serious it was."
Forensics officers from Staffordshire Police combed through the site for evidence and an appeal for witnesses has now been launched.
Police confirmed the 19-year-old, who was sent to Walsall Manor, did not suffer life-threatening or life-changing injuries.
A Staffordshire Police spokesperson said: "Staffordshire Police is appealing for witnesses following an armed robbery at a business premises in Brownhills earlier today (23 July).
"At around 4am this morning , a gang of masked men forced their way into Midland Safe Load Indicators Ltd on Watling Street armed with weapons.
"A 19-year-old man was shot three times with a gas gun and was taken to Walsall Manor Hospital. His injuries are not life-threatening or life-changing.
"The offenders stole a van from the site along with multiple sets of keys to other vehicles.
"An investigation is underway. Anyone with information is asked to call Staffordshire Police on 101 quoting incident 91 of 23 July."