For mediocre quality pellets like Crosman or Daisy, sorting by head size may indeed improve groups. For example I can’t tell you how many times I’ve gotten Crosman pellets that form two distinct groups. I’ve even seen it from their boxed pellets that are supposedly from a single die. Measurements said otherwise.
However I’ve never managed to get results as good as simply using higher quality pellets from JSB/Air Arms, H&N, or RWS.
Another observation that came as a surprise to me was that there is no such thing as an ideal head size for a particular barrel. Most barrels will have as good of a chance grouping well with, say, 4.49mm as it does 4.52mm. As long as it isn’t so small that it tips in the bore or so large that it extrudes and smears against the grooves, it’s a suitable size. Whether it groups well will be at the mercy of harmonics, the quality and consistency of the pellets, and other factors.
Granted, it may well be that sizing pellets will yield benefits by correcting for inconsistencies in the pellets that otherwise would cause variability, but the specific size down to the hundredths of a mm is less important.
Note the same is not true of slugs. They must be sized just right for the bore. The primary reason is because there is no thin skirt to expand and form a proper seal. The slug’s full length is responsible for engaging the rifling in a manner that prevents blow-by but simultaneously does not create too much friction.
