Why no washers? The little bit of radius at the root of the shank of
the bolt needs somewhere to go, yes? And may I assume you're putting
these bolts in a reamed hole?

You'll find the bolt will bury itself into the aluminum or magnesium and
that little radius will tend to
squeeze the metal onto the shaft. Try it on a sample and have a look, the
bolt will be hard to get back out of the hole also.
A radius on the nut can do the same.

Cheers Cam

Ps. maybe not "aeronautically correct," but it works.