What Bart said...
Thinner rod may be more likely to flex, contact the side of the bore and pick up "stuff" that can get embedded in the rod, esp. if vinyl coated.
So when I'm determining length I need to figure barrel + action + guide + a little extra correct?
How much extra length do I need? I believe I've read an extra 4 inches. Is there any harm in getting a rod a little longer in case I get a longer barreled rifle down the road? Or should I cross that bridge when the time comes...