That might be because the bore guide is designed to be a snuggish (technical term!
) fit to the cleaning rod and so may well be much smaller than the bore. Try fitting the bore guide to the rod before you screw on the brush and then carefully insert them both into the back of the action. Works well for me!
The fixed centre of the plastic brush is a lot smaller with longer bristles which allows it to fit.
The only time I've been recommended to go down a size with bore brush we when I bought some WipeOut bore cleaner and Robert formerly at HPS recommended using a 22 brush as it allowed reversal of direction in the bore and so aided the foaming action without pushing it all out!
Hope this helps...
JohnE