I agree regarding the burr due to the reamer.
In this case the 6BR and the 22-250 were done by the same gunsmith who is a highly respected benchrest shooter and gunsmith, and the 22-250 reamer was new, as I ordered it for this job.
I am not blaming the barrel maker, just stating my preferences based upon my limited experience.
Even a new reamer can cut a poor chamber. Depending on the maker of the reamer we will run it into a scrap barrel and see how it cuts, check the finish and check sizes. Been down the road where a brand new reamer wrecks the barrels!
Fouling? Bullets can cause copper fouling besides a rough throat. The jackets will vary from lot to lot. Also velocities can cause fouling to go up as well. The .22-250 is going to push the bullet faster than the 6BR. You have a lot of differences between the two and not just the barrels.
Some copper in the bore is normal. As long as it's not causing an accuracy issue don't sweat it to much. If it's causing an accuracy problem then you have a problem but what's causing it can be several different things.
Later, Frank
Bartlein Barrels