What causes fliers and how do I troubleshoot inconsistent groups?

Look at variables in this order: pellet (try a different tin or brand), hold (especially on springers; verify artillery hold consistency), scope (check ring and base screws for movement, parallax setting, and reticle drift from too much elevation adjustment), and gun (cracked stock screws, loose barrel pivot, leaky valve on PCP, dirty or over-cleaned bore). Shoot from a solid rest with a known-good pellet and identical hold for every shot. Most bad-gun problems trace to pellets, technique, or scope mounting.