From your post I'm presuming that Its not grouping but the hole in the target that you are asking about. It is an issue with co2 powered pistols that the pressure drops as you shoot, and it is likely that this is the cause of ragged holes in the target as the velocity of the pellets drops as you shoot. compare your first shot with your last shot, or if you can, borrow a pcp pistol and try the same pellets ,and then compare results, hope this helps
Steyr LG110 Hunter,AA410 in Gary Cane stock,HC, Steyr LP50,Morini 164ei,Morini CM84e,Anschutz 1417 thumbhole,Rimfire Magic 10/22,Anschutz 1913,Rieder and Lenz Z2,Keppeler 6mmbr