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The bolt / screw thing used to be (back in the 70s when I was an apprentice) heads that were hex (spanner) or slot (screwdriver) respectively, with machine screws often being threaded full length and bolts having a plain shank, but this was never a hard and fast distinction.
Google image search for 'm6 30mm socket cap' gives more full length threads than not - I guess it depends on your supplier as much as anything?
MrE
Interesting.
I thought it was to do with whether is had a point (to be screwed into something) or flat (requiring a bolt)
Nonetheless, a 20mm m6 thing with an allen head has a thread the whole way up. Is a 30mm m6 likely to have a shoulder?