It is believed the men ambushed gatekeepers before attacking five guards stationed at the armoury and forcing them to open the safe.
The gang then walked in and helped themselves to the weapons before fleeing.
No deaths or injuries were reported during the robbery.
But an army veteran, who spoke on condition of anonymity, believes it was an inside job.
The 65-year-old, who was stationed at the base for many years, says it is impossible to enter the base or even force your way in without it being arranged.
“The guards at the gates themselves are military. Do you think it would be so easy to get past them, then head straight for the armoury, overpower those guys without injury and still get away without a scratch? Hulle lieg. This is an inside job,” he claims.
“Somebody told them when and where or someone helped them from the inside. How can they walk in and just walk out like that. No man, who do they think is stupid here?”
The retired sergeant said there had been robberies at the armoury in the past, but the skelms were always caught and it was usually someone inside the military.
“You cannot know where to go if you are from the outside. I would bet my life that the culprits are army or ex-army,” says the former officer.
SA National Defence Force spokesperson Brigadier General Mafi Mgobozi confirmed the robbery, but had no other details as the incident was still under investigation.
In August last year, military equipment, assault rifles and hand grenades were stolen from the Simon's Town Naval Base.
Three men - all sons of naval officers - were arrested and one, Duncan Gouvias, is serving six years in jail for burglary and contravention of the Firearms Control Act.
Police spokesperson Captain FC van Wyk confirms the investigation is ongoing and there have been no arrests.
He confirms: “An armed robbery case has been opened and it is under investigated by SAPS Provincial Detectives.
“According to information the incident occurred on 14 April at 3am at an army base in Harare. Five armed unidentified suspects perpetrated the robbery. Circumstances surrounding this incident are under investigation.”