7 1/2
Phil, I doubt that you have been shooting long enough to remember Winchester (handset) traps, but it was a daily ritual to take a piece of wood (the handle of a hockey stick in Minnesota) with some sandpaper tacked to it. The abrasive was run along the arm to "cut the glaze," "clean the paint off the arm," and ensure that the target got "enough spin."
It was done everywhere. It was the only way to get enough spin.
When Ron Baker and I were tracking targets with our high-speed cameras we found:
1. All targets (we tested - White Flyer, Midwest, Remington) spun the same off the same trap, about 2000 RPM when set to fly with a speed which a radar gun reported as 42 MPH. Faster targets got more spin.
2. Pat-Traps and GMV machines spun them the same. Another didn't. But the difference was small, certainly inconsequential to breaking a target since slowing the flight of a target slows its spin too and slow targets break fine.
3. In two tests, months apart, I sanded a veteran arm on a Pat-Trap the way we had always done the hand-sets. This was after the arm had thrown tens of thousands of targets without any care.
There was no difference in the spin imparted to target before and after the sanding. The birds spin the same off the old, glazed, non-maintained arms as off the tuned-up, cleaned-up ones.
In other words, Phil, the advice you were given makes perfect sense and seems even more convincing with that wealth of detail - I mean, look at it, the nice separation of 8's & 8 1/2's from the more energetic 7 1/2's as well as the reason why - was just made up. It's a perfect example of gunclub folk-physics that sounds like it was based on some tests or something, but instead has just been handed down from expert to expert at the club for generations.
Neil
Have you heard that you need 7 1/2's for the second bird in doubles "because the bird is spinning less due to being encountered later in its flight, after air resistance has bled off more of its spin"?
That was the most surprising discovery in Ron's and my tests. The spin of the bird does _not_ decrease in its flight, at least not enough to detect, and that's to an accuracy of about 1 in 100. You may check that at the link above.
So you can shoot 8's at both birds in doubles, too, though we use 9's for the first because they spin "top to bottom" though the air as they do when they are rolled down an incline and so sometimes are 6's and the extra kinetic energy ensures a break.
Neil