The teenager was walking along the cycle path near Kestrel Way, Luton, with two friends just after 6.30pm when they were approached by a black man, aged between 20 and 25-years-old.
The man asked the group where they were from before showing them a small handgun in the waistband of his trousers.
The teenagers ran off in different directions and the victim heard a noise and suffered a wounded thumb, which later required hospital treatment.
Detective Constable Rachel Lydon said: “The victim’s friends said they heard laughing as they ran away, but this incident has had a very real effect on the teenagers involved.
“If anyone has any information or saw anything in the area, please get in touch with us so we can find the man who did this.”
If you know anything about this incident, please contact Bedfordshire Police via the online reporting centre or by calling 101, quoting reference number C/12492/18.
Alternatively you can call Crimestoppers anonymously on 0800 555 111.