so Im getting my AR soon and was just wondering about mounting options for an optic.

The upper is a flattop with a ARMS BUIS and front sights also, if I wanted an Aimpoint and to co-witness 1/3, would I need some kind of riser or cantilever mount?

www.ar15.com/forums/topic.html?b=3&f=18&t=280593

I love my LaRue LT150 for my Aimpoints. It's 1/3 co-witness mount, but is actually a bit taller than the 1/3 co-witness ARMS #22M68.  
It makes for a "cleaner" 1/3 co-witness sight picture. It will work just fine with your ARMS BUIS.

And of course besides the height, the LT150 features a machined barstock QD lever instead of the weaker ARMS MIM lever.  It also has adjustable tension to fit tightly on any receiver.  The ARMS mount has fixed tension and will often move front to back on the receiver.  And I'm even a fan of LaRue's built-in spare battery compartment.  Oh and don't forget the best customer service on the planet.

LT 150



ARMS 322M68 for comparison