Well, to CLEAN one,, you just have to use an oversized brush & a lot of elbow grease.
But,, you MAY need to look at WHY you have a buildup. if it's rough,, or your gun is out of time,, or whatever,, it may not hurt to have a pro inspect it.
If it's just rough,, a smoothing of that area, or even a change in angle with a forcing cone reamer might take care of the problems.