Dirty barrel and how they shoot ?

 
Ok just a little something to chew over. How dirty does a barrel  get before it starts to shoot different?   I know there will be many factors as to when,why, how much, bullet impact point, copper & lead fouling etc. I thought I would throw this out and see what everyone's experience has been.  Do they shoot high,low,left or right .

I have some rifles that will shoot well with 100+ rds ran through them and some that want TLC after 10-20 rds .

My 22-250 which is my main range gun, seems to prefer 50-100 rounds through it to start grouping its best.

In the past few years I've started to document my CB shot, how many rounds have been through the barrel since last cleaning, how it patterns with rapid firing, etc. Things like that, to get a better understanding of how my rifle will work in different conditions.