I think most of the type blades you are using are made from steel with a high boron content and heat treated.
While there are a number of different steels being used for different gun barrels Idon't thinkany are heat treated
as this would cause problems with straitness. I am suprised at the different results but after stoping and thinking about itbecause of the way they are used it makes sense the as exposed to moisture as mower blades are they would be alloyed to resist rust, Lee