what causes a PCP rifle to dual-group?

[darryl]

Hammerli Pneuma Elite 10. I and another member are experiencing the same phenomena. Start shooting a 10-shot magazine and see two distinct POIs. Anyone ever encounter this before?
thank you,
darryl

[Ribbonstone]

Will usually look for one of two things (although it could be both...but that hasn't happened to me yet).

Something is shifting.  Could be the metal to the stock, the breech to the air tube, the barrel to the breech.  First thought is to after the barrel bands...but its not often the main problem.  Still, if you've removed the band, put it back.  IF you have it on the gun, then remove it.

Pellet is occasionally getting chewed on the way to the muzzle.  Could be the mag feed or a burr in the transfer port area.

[darryl]

Thanks Rib. I only have one band on the rifle now; it came with two. I have to have at least one because the LDC is so heavy it would otherwise droop the barrel to where it contacts the air tank (really soft barrel). I've come across some odd things with this rifle. I may have to order another barrel band since I "modified" the second one. Also I've seen where replacing the transfer-port o-ring solved a couple of problems but added a new one: two of the magazines no longer fit! That may have something to do with the replacement o-ring being thicker than the original. It also had a smaller id, but that id was approximately the same diameter as the TP opening, and it fit in the recessed area of the receiver. It might be causing the receiver to bend a little. I'd have to order the  correct replacement from UMAREX or sand the replacement down a bit.

Forgot to add that the grouping is now tighter, but it still not worthy. At least the two-group thing now just looks like nearby fliers rather than two distinct groups.