Royal Mail are normally very good with loss & stolen claims. Regardless of you mentioning it is gun related what you can say in the claim form that is was special wood which was sent to either be used as air rifle stock or a hand crafted walking stick etc. what they mainly look for is proof of purchase. If you can provide that or know of a friend in business who can provide you with that, should settle your claim.
"If your not prepared to be wrong, you'll never come up with anything original".