I have some rack rails that seem quite poor at accommodating my square hole rack. The M5 screws are designed to pass through the rack holes -- i.e., the rails aren't designed for a threaded-hole rack ...

I've used them for ages but never had to buy them. Is there any practical difference in the various sizes of screws and cage nuts? M6 seems to be the prolific choice, but I see 12-24 and 10-32 are ...