Thought I'd give my ten penneth for what its worth.
A reference pellet used by the marshal is out of the question as no two rifles seem to like the same pellet / batch / dai. But again, the law states that a rifle must not achieve a muzzle energy oveer 12ftlb using any commercially available pellet. So let's chrono using rabbit stoppers.....!!!
I don't think so.
The chrono appears to be the same 1 of 2 used at all events, so its unlikely its the chrono itself.
I know from experience that the chronos use a lot of power. Ergo its probably the battery failing and not the chrono itself.
So how about randomly selecting compettitors via the raffle numbers system at the start. Say 10% of them and the marshal takes a pellet out of the shooters holder and inserts into the barrel. Two out of 3 runs over the chrono must pass.
Chrono doesn't need to run off batteries for long periods of time then. Random testing works in most sports so why not HFT ?..
Just my opinion and possible solution.