Had I been in a situation like Simons I would of taken the following approach this without delay:
Firstly if there are witnesses available who saw the individual they should come forward as tampering sports equipment not belonging to the person on the day of a very important competition itself is wrong and the person should of been banned to take part on that day and if any damages caused then he would be liable to pay for them or the matter taken to the small Claims Court this with an invoice of cost of repairs.
Secondly rifle being unattended does not give anyone the right to help them self's to private property/goods this without the owners consent. Now had the kit been stolen and no witnesses were available then it would of been a lesson learned by the owner not to leave valuables/equipment in question unattended.
But had the unattended equipment been stolen and the thief was found then the Law for such act speaks for itself.
Last but the least it maybe Simon when you went for a break leaving your kit there you may of had a Pint on the side and came back and sat on the Rifle......