I recently purchased a new Bushmaster XM-15 (20'' barrel) A3 Flat Top (detachable carry handle) and was wondering what some suggestions were for an effective system to carry it. Single point set up is much preferred.

Thanks ahead of time.

That's a lot of rifle on a single-point sling....you sure you want to do that?

Go with a good two point. I am partial to the Blue Force Gear/Vickers padded VCAS.

yeah this

20" is looooong for single point

Top Sling adapter and use 2 points.

Yeah I got that feeling too. Being new to ARs, (qualled on an M16 but that was the extent of it) I'm also new to carry techniques and movement. I'm just seeking something will let me transition easily from rifle to sidearm and still allow decent mobility.

This looked like  decent example.

http://www.youtube.com/watch?v=hI9HmH-BtAs

Now I definitely understand that 20'' vs 16'' is a big jump as far as ever being mobile. But I really liked the idea of 4 extra inches of velocity/powder burn/less chamber pressure. I'd still like to be able to practice with the system I have though on being able to transfer to a secondary.

Three of my 20: fixed stock rifles are set up to use either single point or two point slings.
This gives me the option of choosing whatever sling I deem best for the particular use intended.
Singlepoints are best employed for CQB/Urban operations and two points are far and away best for rural patrol. field use.

I use DPMS A1/A2 single point adapter in front of the stock and use CSM single point slings, very effective but you must use the singlepoint with a snap hook as the adapter is only an eyelet and does not feature a fullsize sling slot.
It is also designed to place primary carry on the right side of the body with the eyelet located on the left side of the weapon so if you are a southpaw you may want to look at one of the ambidextrious adapter plates.
It also requires the use of a longer A2 style buttstock screw which is included in the kit.

I also use Vicker Blue Force Gear and Spec Ops Master Blaster two point slings that attach to the standard buttstock sling loop and I use Colt side sling adapters on the front.
These are both fairly simple but well made slings that can be installed and removed quickly yet hold to the weapon without worry of a fastner failure.
The Spec Ops sling has a bungee section for additional sling stretch during weapon strikes, the BFG does not.
I don't see any advantage here because what stretches also snaps back, meaning you might meet Mr. Weapon face to barrel.

A good two point sling is plenty long enough to allow over the shoulder ready front carry and is just as fast to drop the weapon and transition to the sidearm.
They also spread the weight of the weapon over a wider area making them much more comfortable for long term carry of the weapon.

Singlepoints really do work best with 14.5 and shorter barrelled weapons but they do offer the advantage of immediate weapon transition from right to left side without having to remove yourself from the sling and are super great for pie slicing in the confines of a building or narrow walkway..

You can keep both slings types in your kit and switch between them as necessity and mission requirements dictate.
This is one of the big advantages of the M16 rifle system, Modularity!

I went with padded VCAS...YMMV.

I'd say you're right on the money.

My only single point is on an SBR, although I am considering a Magpul MS2 or 3 for my carbine.