IMHO
Legally this is law of tort not sale of goods act. If it could be proven that the seller had sold an over the limit gun then the contract is frustrated and the buyer is fully entitled to his money back.

The difficulty is in proving that the gun was over the limit on the day of sale and it wasn't sheer misfortune that it went belly up shortly afterwards.

Personally speaking I'd be paying the cost or taking the gun back if I'd sold it. Common sense dictates that it as defective at point of sale and the seller has a moral responsibility to his fellow shooters to do the decent thing.

Whilst this may not be financially sensible to pursue through the courts it is a damning indictment on the seller if he washes his hands of the issue. I personally feel the buyer is being MORE than reasonable offering to pay half when he's bought a dodgy gun.

Funny thing is I've met Aquarius and he doesn't seem the sort.