At what range are you trying for 1/2” groups ?
Have you shot at longer distances to see if your gun is flinging “curve balls” ? This is not uncommon, several of mine do this, bb’s are more of a pattern than a group, and it seems to only get worse as velocity increases.
Best I can tell, the problem is the bb itself, being undersize, it bounces off the walls on it’s way down the tube, and whatever it contacts last before it leaves determines the spin on the bb.
If you can either eliminate or control the direction of the spin, you may get more consistency.
Here’s a couple of ideas; we know that oil in the bore is a no-no, but I have used graphite spray on a patch (bore cleaned first !) to coat the bore. Graphite is much slicker than steel, so has less ability to impart spin…this has worked in a few, but not all cases, and only after a couple hundred rounds pass thru it.
Another possibility is some kind of “hop-up” device. You can’t rifle a shot tube to get a consistent rotation, but there are ways to impart end-over-end spin, if you can keep this consistent it should help. I haven’t had time to explore this concept myself, but some
others (on this very thread !) have.
Best of luck !