Many people will run out and buy a scope to mount on their rifle without first realizing there are many issues to buying and shooting a scope gun. To start with, you need to spend some serious money to get a good scope, and then you need to learn how to shoot with it. But, all the money in the world won’t automatically make you a good shot. That takes both practice and study. If you can’t fairly consistently be on target at 300 meters with A2 sights on an AR10/AR15 or standard sights on an M1A, then leave the scope on the shelf. You’ve got issues that a scope won’t fix.
The first AR-15 I owned was extremely accurate with the iron sights. I bought a Colt 4X scope and a mount that attached to the carrying handle with a large thumb screw.
I was disappointed that it was less accurate with the scope than with iron sights. I eventually learned that there was a bit of play between the upper and lower receiver. The weight of the scope caused it to move easier.
I later learned there is a cure but at the time did not know.
$200 Leupold 3-9x40. .