Quick, daft question: I've been eyeballing the installation of some 20mm chinese linear rail - similar spec (on paper, at least) to Hiwin, who identify that the 6mm mount holes should be bolted down with M5 bolts.

Having dry-fit these, an M5 feels sloppier than a night out in Essex - I can appreciate that it offers a little wiggle-room to align the rail, but 20% undersize? I've got various M6 cap bolts - some of the heads fit, some don't, and the M6 thread aligns very securely within the 6mm mount hole; whereas M5 gets lost somewhere in the bolt-head recess on the rail.

Before I order the bolts I'm just looking for a positive noise that M5 is correct for 20mm rail?