Mine won't do that. 14mm@50 yards and 15mm@55 yards, one holes at 36 yards, all outdoors. The 50m test card was 15mm. I think its likes JSB exact 8.44gr 4.52 over express or any over head sized exacts batch no 39000012 (2) seem very goods washed and lubed.
I now give the barrel a very light/gentle clean every tin of pellets.