G'Day .500 Nitro,

I'm not looking for a Verbal Stowch, I just don't like to see well engineered firearms damaged.

The problem with pushing oversize Bronze brushes through a smaller bore is that you can cause unnecessary wear or possibly, damage to some of the critical areas of a Rifled bore. By this I mean the Throat but in particular, the Crown of the barrel.
Can I suggest that you get a large caliber Bronze bore brush and squeeze it between your Trigger Finger and Thumb! The compressive strength of multiple strands of the Bronze wire in a brush is quite high and actually, not at all flexible! And if there is little or no Give in the wire of an over size brush, something else has to Give, and that just may be the Crown of your barrel!

Hence my original Question, "What size Bronze Brush for a .404 Jeffery"?

I measured a Dewey .44 caliber Pistol Bronze brush and it miked in at 0.480" Dia!
So I will do some additional Miking of other caliber brushes to see what may be a bit closer to 0.430". Maybe that's a .410 Bore brush???

HooRoo
From
Homer