I have four .223Rem barrels, One bull, one with 1 in 9 twist, and two of the .223Rem barrels with a 1 in 12 twist (ejector).
I had a problem getting one of ejector barrels with a 1 in 12 twist to shoot a good group at 100 yards. I started shooting it about a year and a half ago, and for some strange reason, I couldn't get it to shoot. It would shoot a good group at 25 yards, and when I moved it to 50yards, it was still a decent group, but a little bigger. When I went to 100 yards, I couldn't find the holes in the paper. So I went back to 25 yards and repeated the same procedure, and got the same bad results. I did this a few times, before I finally packed the gun away, deciding to retry sighting it in later on.
Well this year after hunting season ended, I dug the gun out and was determined to sight the gun in. Needless to say, I was having the same problems. Sight in was excellent at 25 yards, passable at 50 yards, and horrendous at 100 yards.
So I went thru all the steps, checked the tightness of the scope bass screws, the screws attaching the rings to the scope base, the scope ring screws, added "O" ring, etc.. I then thought it was a problem with my scope, but I knew this scope (a no name 2-7x) was good. I brought the target back in to 25 yards, and proceeded to again try to sight the rifle in. Same results. Then I caught a slight hesitation between the time I pulled the trigger, and the time the round went off. I tried it again this time, watching the hammer fall when I pulled the trigger. I noticed then that the offset hammer spur, was making contact with the base of the ocular lens. I unloaded the gun, then proceeded to pull the hammer back and dry fire it with me holding the hammer while it was falling. It seemed to clear the scope while I was doing this. I then held it at an angle and looked to see if it was rubbing against the scope. There was light between the hammer spur, hammer, and the scope. I cocked the hammer, and pulled the trigger on an empty chamber, while watching the hammer fall, and noticed that the offset hammer spur just brushed the scope as it fell. It was just the slightest contact. But I guess this was the culprit. This minor contact didn't affect the POI at close ranges, but it got magnified the further the target was away. I decided to take the gun home, and use the handy Dremel tool on the edge of the offset hammer spur, to lower the height of it by a hair. When I got home, my light bulb which was on dim finally got brighter, and I decided to turn the off set hammer spur over, since it was thinner on the bottom side. I got the right hex key out and performed the operation, and lo and behold, there was now no longer any contact with the bottom of the scope.
I'll have to wait until Monday to take the gun back to the range. I think I finally solved the problem of not being able to sight in this barrel. I hope.