This will be the first scope i have ever used and i am anxious to get the rifle finished ,but need help with some questions.
I am looking at the Burris PEPR ,Aero spr or Primary arms as these are in my price range if they will work.

I have buis that are  .4375" tall while folded down and a Vortex strike eagle scope on a Bcm 20" upper

Will these mounts be high enough for the scope to clear the BUIS and since the mount will be in front of the BUIS should i go with extended 1",2" or rings ?

Thanks

Which back up sight brand? Also the the extended scope mounts are for people who use less length of pull therefore their stocks would be collapsed in more. Or those who prefer closer to nose to charging handle style shooting like the military teaches.

Aero is 3.36 oz and the PEPR is 8.3-8.7 oz...Aero would be my choice

For a fixed mount, go with the Aero or consider the Leupold Mark 2 IMS one piece  fixed AR mount, similarly priced.   The PEPR is too heavy and bulky for my liking.

To determine whether your scope, mount and rear back up are compatible, there is a formula.

1.  Look up the mount's specs.  What you want to know is how high it is from the top of your rail to the centerline of the scope rings.  Write that number down.

2.  Look up the outside diameter of the scope eyepiece.  It will usually be on the scope manufacturer's website.  Write that number down.

3.  Now, take 1/2 of the number you wrote down in step 2  above.  That is the amount of your scope eyepiece that will be below the centerline of the rings.  Write this down.

4.  Subtract the number you got in step 3 from the number you got in step 1.   The result is the amount of clearance you have from the bottom of your eyepiece to the top of the rail.

If that number is larger than the thickness of your folded rear sight your mount will work.  I recommend you add at least 1/8" (.125") clearance for good measure.

Example:

Areo Precision mount spec is 1.5" from top of rail to centerline of rings.

Strike Eagle eyepiece is 1.7' in outside diameter.

1/2 of 1.7" = .850"

1.500" minus .850" = .650" clearance between bottom of eyepiece and top of the rail.

Your rear sight is .4375" tall folded.

It will comfortably fit under the scope with that mount.

You can do the same thing with any other mount, if you look up its height from rail to centerline of rings.

I had a PEPR mount and it was heavy and wasn't aligned well during machining. I returned it and put an ADM Recon mount on and after returning my scope back to zero with a lot of reverse adjustment from the PEPR mount it is great. I put a Vortex PST 1-4MOA and Troy Battle sights on it and everything fits great.

Thank you all for the detailed input .  
I think i'm going with the Aero mount i found the non spr version (since all the specs seem to be the same except offset) for $68 that will give me a little extra to buy the wheeler fat torque wrench also ...

Below is a picture of my Aero Precision EXTENDED Lightweight mount.  Notice how the EXTENDED mount moves the rear ring 1" forward.  AP also makes an SPR mount which moves the rear ring 2" forward.  

I'm very happy with the EXTENDED mount with the Strike Eagle.