In springers at least, performance - both power and accuracy - can be markedly changed by the depth to which the pellets have been seated. In the 80s John Walthers (The Airgun Book) did a big experiment with a large number of break-barrel rifles and found that finding the correct depth to seat a pellet can yield good increases in power and accuracy. The throat of the breech obviously has a role to play, but the dimensions and texture of the individual pellet types also feeds into where exactly the sweet spot is. If a pellet is not seated deeply enough it may lose efficiency by being stuck in the breech too firmly, if it is seated too deeply it might provide too little resistance and launch before peak air-pressure has been reached.

This is one way in which the pellets are 'sized', by pushing them into the bore. The problem with the two or three commercial seating devices is that they were non-adjustable; people just had to push the pellets in and that was that. An adjustable one, or one which is made for an individual pellet/rifle combination might yield significant increases in performance.

Of course, Walthers was dealing with the dodgy pellets of the 80s, but it might still be a valid experiment for us tinkerers to have a go at; try making a small seating device from a small bolt, a couple of nuts and a washer (obviously file & sand a nice curve on the seating tip) and then try seating your pellets at various depths. You might shrink your groups and increase consistency for the price of a Kitkat.