OK I was going to be clever and use a slitting saw to make rail grooves on my bolt extension. WOW what a mess! The saws aren't able to cut into the metal for very long before (A) getting super-hot super-fast, and (B) dulling themselves in just a few passes. Destroyed two saws in about 15 minutes then gave up and finished with the dremel and a small file.
Is there a special kind of slitting saw that is intended for steel work vs. plastic/aluminum work? That's the only thing I can think of - that these aren't really for steel, rather softer materials.
I got a handful of these used off of eBay for about $1.50 each. Yes, they are fine tooth.