(this is not about saying bubble levels permanently attached to scopes are trash, that's something very different than initial scope mounting and alignment)
Whenever any topic about scope mounting comes up, anything even tangentially related, half the people commenting don't even understand what a properly mounted scope is. I blame the tools that have come out and are so popular nowadays (and such trash). Because of these tools, most people, even on this sub, will tell you a properly mounted scope is level with the rifle because that's what they did with the tools. This is wrong, and it makes people ignorant to what is actually going on and important, which makes them ignorant of other foundational parts of long range shooting. Please stop this.
A correctly mounted scope has nothing to do with a level rifle, zero, zilch, fuckall. Any tool that attaches to, places anything on, or has a level for your rifle should go in the trash. You are not trying to level your scope to your rifle. Hell, you aren't really even trying to level your scope. You are trying to bisect the bore of your barrel with the elevation travel of your scope. It's that simple, everything else will fall in place. First, we will learn WHY then we will learn HOW to do this (hint, it's not with overpriced tools bolted to your barrel/reciever/turrets etc).
What level doesn't matter? No, that's wrong, you're an idiot, what do you mean? Lets ponder what would happen if your "level" scope was off to the left side of your "level" rifle by a couple inches (think M1D). Use your fancy bubble level mounting kits on this setup, go ahead. You zero in at 100 yards, but in order to do so, your scope of course has to be windage adjusted horizontally to the right, in toward your barrel. Great, perfect hit, now dial your elevation to and shoot at 600 yards, wait, what the hell you are shooting WAY off to the right now? Well your scope elevation is not bisecting your bore, those fancy tools didn't do **** to properly mount your scope and while having both the scope and rifle "level" kept your dialed elevation true, the misalignment totally messed your windage up, congratulations, you just missed your target. This is an extreme example meant to drive home the point that level doesn't matter. These tools do not aim to properly mount scopes and they only get close to doing so by chance. This example is horizontal misalignment.
OK ok I get it, elevation travel needs to bisect the bore, but I spent $100 on these fancy bubble levels! What if;
My scope is perfectly centered in the rings (probably)
My ring mounts perfectly center the scope on the rail (nah)
My rails is perfectly centered on the receiver (good chance on a custom action, not a factory one)
My barrel was threaded and mounted perfectly centered in the receiver, and the bore is perfectly centered in the profile of the barrel
Also, my turrets/whatever I am using for scope level is perfectly level with my reticle.
BOOM, now If I level my receiver and level my scope I'm good right, right?.... Lets pretend I say yes for a second here, are you comfortable making all those assumptions? Because you are going to be wrong a lot of the time. But yes, assuming you can read a bubble level perfectly at high precision (you can't) in this case the tools would achieve a scope mounting with elevation travel bisecting the bore.
Oh wait, did I forget to mention?, it's possible that your scopes elevation travel might not actually travel level with your reticle and/or scope bodies "level" and even though you leveled it, you didn't achieve bisection? You just missed your target again, woops, my bad. This doesn't happen often, but it does happen, and you had no idea, because you used a tool that doesn't aim to do what it's actually supposed to.
So, what is the right way to do this? Simple, throw out all those shitty tools, or if you really must, use them to get the mounting close before proceeding.
Zero at 100 yards
Draw a 40" tall PLUMB line with a target dot at the bottom
Align your vertical reticle with the plumb line and shoot the target to confirm zero.
Dial 30 MOA of elevation, or the most you can while staying 5-10 MOA from your maximum (maximum elevation can cause windage shifts as an off centered tube is forced to the center of the radius)
Make sure your vertical reticle is still aligned with the plumb line and shoot again.
Your bullet should hit 30" high. Oh what's that? It impacted off to the left or right of the line? NO ****, because those crappy tools didn't align your elevation travel to the bore like I keep telling you, so with your reticle/scope level with the plumb line you effectively horizontally offset the scope like the above example of the M1D. How do we fix this? If you shot to the right of the line rotate the scope counter clockwise, and vise versa, repeat procedure. Once you are on the line, congratulations. Now that your scope is aligned.
Your shitty tools may or may not show that the scope is correctly mounted at this point, if they do that does NOT mean they are precise and you should use them. They probably would have shown proper alignment when you were shooting 1 MOA off to the sides of the line as well, they are not precise instruments, throw them in the damn trash.
But, I used my fancy tool, did the above, and I got really close to the line the first shot! Congratulations? Would you also want to be patted on the back for hitting that duck somewhere behind the duck you were actually aiming for? Achieving a good result by aiming for a different result is not to be praised.
Also to note; this is basically same reason that we try to keep scopes as close to the barrel as possible. The difference here is that the horizontal misalignment (like the M1D example) starts from your zero distance to your actual target distance, where the misalignment caused by scope height takes effect from your dialed elevation distance to your actual target distance (which is always a much smaller factor, unless you regularly hold over 1000 yards of elevation). That's why we can get away with scope 2 inches above the barrel, but you would be totally screwed with a scope 2 inches in horizontal misalignment. Even a couple MMs of horizontal misalignment is over a tenth of an MOA.
If you want to argue with me that this method cannot work "because math" go argue with Horus page 22 instead please. Much obliged.