I am working on a new actor (based on the coalition heavy) that also happens to have a helmet. It's added the exact same way, by an AddAttachable to the head code. It looks fine initially, but as a match wears on, many of the helmets start to move, to a point where the hair of the head is sticking through the top of the helmet. I don't think the vanilla helmets move this much, and I pretty much mimicked their settings with the exception of a nudge here and there to align things.

Another question is: what is the difference between spriteoffset and jointoffset? And for things like heads, there's a spriteoffset for the head itself, and another spriteoffset when the head is attached to the main actor. Is there a reason for using two sets of numbers for what is essentially the same thing?

there shouldnt be another spriteoffset.
the spriteoffset moves the sprite itself around, basically so its centered on the axis, which is what it will rotate on. jointoffset is a point on the axis that joins the sprite to something.
attachables should not shift down.. ever. can we have some gifs?

They do shift down . Look in ASC at my basicsoldier's helmet . Every 3-4 seconds its shifting down and left (that way /)
...............v

I would post a gif, but it is something that tends to happen over a long period of time. Maybe a time lapse gif? I'm fiddling around with it some more, the helmet design was really thin (like a biker skullcap) and didn't allow much room for error so I made the helmet a bit bigger. (the shift was maybe 1-2 pixels at the most.) I think that should help.
Thanks for the info on the offsets. They are pretty confusing; I thought they'd be more straightforward.

It's the sprites. Depending on the angle it might look right or wrong. The Coalition guys get around this by having thick, black borders on the top of the goggles and the bottom of the helmet. Try that. And make the sprite a bit bigger so the top of the head doesn't show through.