In @flashpoint's original post, he comments on having a very tight bore when inserting his brass brush down a barrel that is only identified as a 45 caliber Kentucky. My speculation is that the bore may be closer to 44 caliber than 45 and a 40 caliber brush or a 40 caliber jag with a thick patch would work better. Of course he could put the jag in his drill and file the diameter of the jag back to slide down the bore easier.