I measured the OD a while back as I was planning on making a laser mount that would hold the laser right @ the muzzle to see how the laser spot jumped around wildly on the target at the moment of firing.
That project went by the wayside after I decided to remove Gamo's spring preload - the rifle lost about 150fps but the shootability of the rifle increased to the point where I can stand, unrested, and shoot 3/4" 10-shot groups at 25yds, 9 times outta 10.
This is completely adequate for me and I've given up tinkering with that thing - it's as good as it's going to get, I think.
In response to your query, the hole I was going to mill for the laser mount was 0.800", about an inch or so back from the muzzle.
The plastic coating is soft enough that 50 thou either way isn't going to make any real difference.
Good Luck with your project!