A 69-year-old man was assaulted when he discovered a burglary taking place at the gun club in Burgh-le-Marsh.
The incident took place around 2pm yesterday afternoon at a shed to the rear of the Burgh Rifle and Pistol Club in Ingoldmells Road.
Police say the victim had gone to the shed to put equipment away when he discovered a young man, who assaulted him before fleeing.
The suspect is described as being white, aged in his late teens to early twenties, around 5ft 9ins to 5ft 10ins tall, with a slim build and a round shaped baby face.
He had dark short hair and was wearing a grey hooded top, grey t-shirt, black work-wear style trousers and black gloves.
DC Rebecca Latto is appealing for any witnesses who may have seen anyone matching the description in that area between 11am and 4pm.
Anyone who believes they know the identity of the offender is asked not to approach them but to call police on 101, quoting incident 282 of February 1.