The trunnion hole is 7mm unless its been modified. I use a 7mm end mill initaly to flatten out eh barrel so a drill bit will not walk . I then swithc to a drill bit .but with a "standard" barrel pin hole in the trunnion and no hole in the virgin barrel should i mic the hole in the trunnion and get an appropriately sized end mill?
mill/ drill in from both sides is what I do .
there are ready made over size pins for setting head space on used barrels etc .