The Co-op in Long Furlong Drive, Britwell, was raided on Friday evening by two people carrying what appeared to be a handgun and a chisel.
After filling a carrier bag with cash, the offenders got into a "scuffle" with staff resulting in the handle falling off the gun, police said
It is thought one of the robbers may have been involved in a collision as he attempted to escape on a bicycle.
Thames Valley Police said an off-duty police officer tried to follow the offenders and saw one being helped up from the road at the junction of Wentworth Avenue, as if he had been hit by a vehicle.
The force said the gun was thought to be an imitation firearm. The bicycles used in the escape were a gold-coloured mountain bike and a bright green BMX with red handlebars and pedals.
Officers are trying to trace the driver of the vehicle that collided with one of the cyclists following the robbery at 21:10 GMT.
Police said two men, aged 33, had been arrested and bailed until 31 December.
Two men have been arrested over an armed robbery in Slough.