Testing firm 1. Our policy is to test all rifles as taken, with three varying weights of pellets, all under controlled conditions with calibrated apparatus and up to date certificates of conformity. Price per test £1000.
Testing firm 2. Our policy is take all rifles to bits, leave parts out when we reassemble, doctor in any way feasible to try to make the rifle go over the limit, and to choose any kind of projectile from a pellet to a carpet tack. Our chrono is an off-the-shelf model. Price per test £500.
Both are equally valid, so long as they both are seen to comply with the procedures the firms have determined and laid down for themselves (and these don't need to be approved or ratified elsewhere; if a jury is convinced they are reasonable, that's it).
I'd like to think this too was an area for discretion, but when one house is half the price of the other, who gets the work? And who looks more likely to secure a conviction, which is one of the factors the CPS takes into account.