Hi, basically they are a one piece mount that is bolted to your rifle and then the scope mounting rings are attached to it. Those and the Weaver style 1 piece bases have some design benefits in some cases. They allow more choice in how close or how far away you might want to mount your scope. The m.o.a. is the angle they point down which is minute of angle or a smidge more than 1 inch at 100 yds. so a 20 moa gives you 20 inches more elevation but it is just an angle that relates to something and does not necessarily make a rifle more desireable or accurate. I use them on my rifles because I am too cheap to replace my good older scopes with modern long range scopes that have enough elevation adjustment to hit a long range target. Typically most older scopes only have enough elevation adjustment to make them on target anywhere from 500 yds. to 800 yds. I also use 35 moa on my Extra long range rifles with modern scopes. They are just a way of overcoming difficulties unless you plan on mounting military type equipment then there is a groove spaceing difference between weaver style and picatinny even though the terms are very often melted together or slang. Weaver style rings will fit picatinny but there is no guarantee that military attachments will fit Weaver style bases.
