+1 to Jjfuller on post number 12. The scope will see through the front post, and don't need to get high mounts to see over the top. The scope cant magnify on something that close, so it looks right through it.
A 3x9x40 is a great choice for seeing targets way out there. What I don't like about a 4x plus scope is they get to be useless for close range shots, as they need more distance to focus on. if you are in heavy woods or dense brush, a 3x9 is better, and something like a 1.75x whatever is even better. For open range shots, a 4x is great. But a 3x can do pretty much what a 4x can do, except for super far distances and you want to read the made in china stamp!
Here is my 3x9x40 on my AR: