Unfortunately there are many potential causes. Listing just a few:
1. Subtle pellet damage resulting from a burr at the leade, barrel port, or crown. Push through a few different pellets by hand and feel for places where it catches…where you feel a little hitch, not so much referring to general roughness along its length. FWIW my Avenger’s barrel had pretty rough machining which is all but guaranteed for stuff at this price point. See here for a DIY guide on identifying and addressing these types of deficiencies:
Barrel troubleshooting and accurizing how-to, Air guns, Airgun Forum
www.gatewaytoairguns.org
2. Fouling. A rough bore is apt to strip off lead, leaving it behind for the next pellet to pick up and disrupt its balance to send it off course. Deburr the bore’s surface fretting with a polishing compound like J-B Bore Compound. Then for pellets, apply furniture wax to the bore. Or for slugs, try lubricating them. My .22 Avenger barrel claimed the dubious distinction of being the roughest bore I have encountered to date.
3. Clipping. Very, very subtle clipping, or perhaps just turbulence from a glancing near miss. Remove baffles (if present) and the end cap, then retest.
4. Harmonics. Anything and everything associated with supporting the barrel is a candidate. Barrel band, shroud and its mounting bushing, shroud spacer, etc. A barrel must be either fully floated or fully and consistently supported, never something in-between. Any free play to allows things to vibrate or slide or shift is likely to cause problems. For example, the shroud mounting bushing on my Avenger (rifle, not bullpup) was secured with a single fastener and could move ever so slightly. I modified it for a second fastener and the improvement was immediate.
Again this is just a partial list but they represent the more common ones for the particular situation you’re experiencing. That is, where you’re getting quite a few very near the bull but some that are just wildly off.
