The wooden rod for my .40 is 5/16" and is a nice fit.
I have a stainless steel range/cleaning rod that I share between my guns -- bought it in 5/16" also and it is a nice fit in the .40. I like it better than a "flex" rod, especially while cleaning. The sturdy knob-like handle makes it easy to use -- especially the time (or two) that I had to pull a ball.
I recommend a brass bore protector to go along with metal rods, or any synthetic rod that might abrade your bore / muzzle.
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