Actually, the adapter does have a bore size.
The automag bolt is very close to 0.680, so the adapter has to at least clear that. I've got one of Doc's around here; I think it came out to at least 0.685 measured. If I find it I'll see, but for the purposes of the question, you can't underbore the adapter because then the bolt's going to have a bad day.
"Accuracy by aiming."
Definitely not on the A-Team.
I have to disagree. A bore size is just that - the size of the bore. Since the adapter has no bore, it can have no bore size.
It does have a breech size, but that makes no difference on underboring, overboring or bore-to-paint matching.
On twistlock barrels, the bore and breech are permanently connected as one barrel (or barrel back in 2-piece situations). On cocker-threaded bodies (ex. ULE bodies) the breech remains as part of the body, and the bore (barrel) is connected. On TL bodies with an adapter, the adapter serves as a less easily removed breech, to which the bore is again connected.
Breech size can matter though. It's where the ball drops in. If it's too large, it allows the next ball in the stack to partially enter the chamber and you get the wounding-the-next-ball effect.
Or if you have a curiously shaped paintball with a skirt on the backend that you don't want the bolt hitting -- the way to address that is by adjusting the breech...