Re: Shooting prone with gun laying sideways at a 90
You can actually calculate that shot pretty closely.
Assume that we're talking a 100 yard shot, and you have a 100 yard zero on your scope.
Your elevation zero is composed of two parts. One is compensation for the drop from the muzzle to 100 yards. With a .308, that drop is about 2.7 inches or so, about 2.6 MOA or 0.75 mils.
The other part is the mechanical offset of the line of sight from the line of the bore. Assume a sight height of 1.75 inches, or, at 100 yards, about 1.7 MOA or .5 mils.
Add those up, and you get about 4.3 MOA, or about 1.25 mils.
So, if you rotate your rifle to the left, or bolt up for a right-handed rifle, your windage is now going to be your elevation, but it's at zero.
And your elevation zero is now your windage zero, but it's about 1.25 mils to the right of the line of the bore.
So, if you fire at shot at a point of aim of the scope with no adjustments, the point of impact will be about 1.25 mils to the left of the POA, because that's how far the scope is off from the line of the bore, and about 0.75 mils low. (That's because we no longer have a sight height to compensate for - the scope is at the same level as the bore.)
To visualize that take two pencils or pens. Hold one above another. The top one is your scope, and the bottom one is the line of the bore. Now tilt the bottom one up at the front toward the target, to simulate tilting the muzzle up, to compensate for the drop and the height of the scope above the bore.
Now, maintaining that relationship, rotate them to the left, to simulate turning them to make a bolt up shot. With the line of the scope - now on the left - pointed at an imaginary target, observe that the line of the bore is pointed to the left of the imaginary target.
So, if you hold to the right by 1.2 mils, and high by about 0.75 mils, you should be pretty close to the target.
I don't recommend trying to dial that shot, because it will mess up your zero. Just try holding what we calculated, or what's appropriate for your load and your sight height. See where the bullet goes, make the appropriate connection to your hold, and record it.
If you're making a bolt up shot at a distance of greater than 100 yards, it gets more complicated. In that case, you might want to actually establish a parallel bore zero. In that case, you'd just have to hold to the right by the amount of the sight height, and hold high by whatever the actual bullet drop was at the specified distance.
But this is realistically a pretty short-range technique, except as a match gimmick shot, because the position you have to be in to hold the rifle makes it hard to get the rifle stable. If you have to make this as a real world shot on a human target, you shouldn't have much trouble if you hold a few inches to the right and a few inches high. And good luck and God bless you if you do.