Mark Langley was kicked to the ground and along with a customer had to lie face down as the duo smashed a display cabinet and grabbed £10,000 worth of jewellery.
They burst into Watch & Clock Repairs in The Street, Tongham, just before 11am on April 22.
He was serving the customer and saw them coming but could not stop them.
"A car pulled up outside suddenly and the two men jumped out.
"I tried to get to the door to stop them getting in but one of them had a gun. The other chap came in behind.
"He smashed the door with a hammer to get the jewellery and a couple of watches."
Of the gunman, Mr Langley said: "He was waving the gun around and telling everyone to get back in the shop.
"There was a guy outside who tried to stop them and he got clouted round the face, as one of them shouted not to be brave or he'd shoot.
"It just wasn't professional. It appeared more like an opportunity, which is more worrying."
The robbers then ran to the car but one returned for more jewellery.
"I got up but he came back in and shouted ‘get down or I'll shoot'."
Passer-by James Richardson had the gun pointed at him and was struck in the face by one robber as they fled.
He said he knew something was wrong when the car skidded up outside and he heard glass shatter moments later.
"I ran up to the door and shouted but before I got to the door I had a pistol in my face.
"I think I must have disturbed them."
He said one of the robbers hit him hard with the gun, but he was not frightened by it.
"He pointed the gun at me and he was only a metre or so away.
"He clouted me as I was talking on the phone but I could see it in his eyes though that he wasn't going to shoot."
Staff and customers in the Londis store next door were shaken, said sales assistant Lucy Andrews.
"I was shocked and scared. I looked out the door and saw two blokes jump out with masks on. We thought they were coming in here so we all ran out the back."
She said she never thought anything like that would happen in Tongham.
The duo escaped down Poyle Road in a white Ford Fiesta, which was later found dumped in Murrells Road, Ash.
Mr Langley said the items stolen were second hand and identifiable, so it would be "very difficult" for the robbers to sell them on.
A gentleman's 18-carat gold Rolex watch, a stainless steel Girard Perregaux watch and some gold chains were among the items stolen.
Witnesses and anyone with information should call Det Con Mark John on 01483 531111 or Crimestoppers on 0800 555 111.
Mark Langley is pictured outside his shop.