We're talking MOA and mils here brother - different. Each 1/10th mil (0.1 mil) is approx 0.36" at 100 yards. One full mil is 3.6" at 100 yards. Basically, 0.3 mils would be approx an inch at 100 yards. If you had a 0.5 mil shift at 100 yards, that's over 1 2/3rds inches. Damn near 1 3/4 inches when you math it out. In a nutshell, the 0.3 mil shift would be the same as a 1 MOA shift.
Clean bore shots will be different than "fouled barrel" shots. You'll see it alot less on true match barrels, that have been hand lapped - they're just smoother.
Clean bore is CLEAN - you ran all your copper remover through it until there's now none left in the lands and grooves - and all carbon fouling would be gone then, too. We're talking CLEAN. There are all kinds of crags, mountains, valleys and scary shiit when you look down the rifling of a mass produced barrel. I'll bore scope a recording of one and put the video up. It takes copper, stripped from the bullet's jackets, to "fill" those areas and kinda "smooth out" the lands/grooves of the rifling. Once that occurs, the shot groups will be much more consistent. At that point, you have a "fouled barrel." Doesn't necessarily means it's all carboned up - it's "coppered up."