Pellet seating tight.

As I tend to shoot pellets that are hard, on a lot of my guns I've opened the leade a bit with sandpaper-on-a-stick. Usually I use an 150 grit followed by something finer, like 800.
Sand a little, in and out, in and out at about the depth of a pellet, rotating the barrel area and the stick so that no spot gets all the sanding,
clean with an oily patch run through from the _barrel_ end,
test fit the offending pellet and,
repeat until it fits.
So far it has only helped accuracy.
Mike