Ditto on most advice given above. I am no expert, but, this is what I do.
When I mount a scope, I find the turrets half way
adjustment points by turning both elevation and
windage turrets in one direct until they will not
turn anymore. Then I count the number of clicks
it takes to stop again, while turning them in the opposite direct.
If , let's say, there are 100 clicks from one stop point to the other, I set the
elevation and windage at 50 clicks, the midway point.
Then I mount it in the rifle. I do the above, because, being set at midpoint,
it quickly tells me if there is a major fault with the setup when I fire the first couple of rounds. NOT!! that I expect the point of impact to be close the bullseye, but, if I am a couple FEET off, I start looking into a MOUNTING PROBLEM.
The quickest way to get a scope on target, is to put it in a shooting sled, mount, vice, whatever that will maintain a really good stable rifle/pistol confinement. fFre the first round with the cross hair on the bullseye, then, keeping the cross hair on the bullseye, while confined in the sled, adjust the cross hairs so they are move t the centered on the first bullet hole. After that, you should be acquiring impact where you point the cross hairs. Of course some fine tuning may be required.
As to your query on MOA, first, that is an abbreviation for Minute of Angle.
Basically 1 moa is 1/60th of a degree which translates into 1 inch deviation of elevation, or windage, from muzzle to target at 100 yards. So, if your scope has 1/4 inch turret click adjustments,... four clicks would equal 1moa or a one inch adjustment.
Now, as to your question on a 20 moa base, the above info should make it self evident. But, simply put,... a 20moa base would deviate from bullseye impact by 20 inches HIGH. I have never needed one, but, these bases are used to elevate the point of impact of a scope for very long distance shooting by allowing the scope to have the elevation knob set very low ,to get on target at 100 yds,... which,... allows plenty of elevation adjustment for the very long distance shots. Which is where some scopes fall short. Bottom line, a 20moa base gets you more long distance adjustment out of a scope than you probably wouldn't get without such a base. I this type base is being suggested to you, it is a simple but incorrect solution to a mounting problem resulting in your current Point Of Impact being very low.
There is a rather extensive learning curve to properly mounting a scope. But,
most procedures are personal choices, rather than one being right or wrong.
Sadly, most mistakes aren't noticed by some, and I have seen many who have been shooting with improperly mounted scopes for years and no severe problems. I would assume there are books, but, I would ask someone you trust to give you some hands on advice. Mounting a scope, is like riding a bicycle,...once you get it, you have it for life!
JM.02OVOTE TRUMP !!!