"If the rifle is used you might want to mechanical zero the scope. Misaligned mounting and someone putting clicks on it can leave you little adjustment in one direction or t'other."...which reminded me of a tale...
'Way back in the Fall of Aught-One, I ordered a handy little Ruger No.1 RSI in .243 Win from Davidson's and purchased a used Leupold fixed 12x scope for it from a coworker. (Yes, current me laughs at old me's choice of optics, too; that thing wanted a fixed four or low-power variable glass, not a lunar observatory.)
He offered to mount and boresight the scope for me, which was nice. As I left to run some errands and fetch lunch, he had my little rifle in the cradle on the counter and was breaking out the scope ring lapping kit. When I returned to the shop, the lapping kit was as yet unused and he was spinning a ring reamer and standing in a disturbing amount of metal shavings from the scope rings.
"What's up?" I asked, cleverly.
"Here," he said, plopping the Loopie glass in the rings and tightening them down, "Look through there at the collimator and see if you can tell."
Sure enough, the crosshairs were 'way over at the edge of the grid on the old-style ground glass boresighter, and the adjustment screw was pretty much all out of clicks in the opposite direction. This was not good.
After a bit more grinding, it was apparent that we were going to run out of scope ring before we got the thing dialed in, and the base was, in fact, unusable. (We were fast on the draw like that.)
The rings on a No.1 are mounted on a little quarter-rib, which is secured to the barrel with screws fore and aft. My co-worker began to loosen the forward screws and, when he got down to, like, the last the last half-turn on the second one, the front of the sight base torqued to the side off the barrel, pulling the screw free with what my mind insists on remembering as a *ping* but which was more likely a *click*.
With the quarter-rib off the gun and the upper scope-mounting surface held level, it was obvious to the naked eye that the two "feet", the mating areas at the front and rear on the bottom of the quarter rib where it sat on the barrel, weren't machined anything remotely like concentric. In other words, while the "rib" part of the quarter rib was nice and straight, the feet were askew, and tightening both feet down flush to the gun twisted the formerly-straight rib like a piece of rotini.
"I don't have to worry about that!" says the Cletus in the back, at this point in the tale, "I only buy nice guns!" Hey, this was on a rifle that had an MSRP of something like seven or eight bills even back then, right? You could have darn near bought two Remington Wally World specials for what this thing cost.
Ruger handled it well though; we called them and they sent a fresh quarter rib and a set of rings gratis, without even a core charge. I still have the cattywhampus quarter rib someplace, I guess. I think we pitched the rings.