Waksupi,
I am chasing a similar problem with a rifle currently. I can shoot it off a rest after a good cleaning at home and get a group with the balls touching at 50 yds. Looked good on the first group (3 shots), but the following groups opened up to a solid 3 inch group. Bummer!! I cleaned the rifle at the range with patches, not scraping the breech face, and the 3 inch groups continued. Finally, I took a scraper and cleaned the breech face. The residue would grab hold of the scraper so bad that it wouldn't turn if I had much pressure on the rod. Large chunks of residue fell on the ground when I turned the muzzle down and tapped the barrel. Cleaning only the breech face did not get the small group back. I went home, cleaned the rifle, and went to the range the following week with the same results. First group great, remaining groups 3 inches O.D. I think I am not getting a good burn when the rifle fires resulting in the heavy residue build up and causing the accuracy to go away. I am not having any hang fires, but the flash hole must be building up with residue like the breech face. I clean the touch hole better when I clean it at home, and I think everything being very clean accounts for the first group being good. I have been using the remainder of a pound of GOEX that is several years old, and I am going to try a new can of powder next Tue and see if it fixes my problem. The rifle is capable, but I have something going on causing the open groups. First time I have been down this road!
Roger Sells