So After re-reading your initial post, this brings back some experiences with a 10" octagon 45 colt barrel I have. At 25 yards with standard to hot 45 colt loads it would make very nice groups, free hand or off a bench. This was a while ago, but I seem to recall 1-3" groups depending on load, stance, rest..... I was excited and thinking i might use this for hunting. Then I moved back to 50 yards. I was getting 1 foot groups or worse with every combination and quickly concluded this wasn't going to be a hunting barrel.
I never figured it out and went on to shooting other barrels. The difference in group size between 25 and 50 yards was huge, much larger then the expected 2x increase in size. I think it has to be something about bullet stability or the barrel has the ability to randomly hook and slice shots so the external ballistics change.