I tend to leave a LITTLE bit more headspace in 22oz bombers. If nothing else, when you remove the bottling wand, there's a bit more than an inch of room left over from the displacement of the wand. I can't imagine that a small variation would make much difference.
Plus, IIRC, overcarbing occurs if there is TOO MUCH headspace, not too little. Don't ask me why, I might wager a guess that less beer in the bottle leaves less beer to absorb CO2 to be absorbed. I wouldn't wager any money on this, but it seems that I read about the headspace issue and I recall the answer was the opposite of what my expectation would be.
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