I think that that depends on the particular Foam Backer Rod that you're using to make darts (assuming that you're using micro stefans)...7/16" ID wouldn't fit my 1/2" FBR very well, and if it fits yours, I'm guessing it'll be a little on the tight side.DO you think 7/16 ID would be too small or still usable for good distances on the at3k.
Where are you getting your PETG (I'm just curious, never used PETG myself)?