I'll be up front, this is probably my favorite rail right now. As with their triggers, Geissele Automatics craftsmanship and attention to detail is second to none. So far MK4 rail is one of the nicest and functional free float rails out there. The rail is strong, and gives me rail points where I need them and nowhere that I don't, saving weight. Here is the most recent upper I built around this rail.

Manufacturer: Geissele Automatics

Item to be Installed: MK4 13" Rail

Purpose of Item: The Geissele Super Modular Rail MK4 (SMR) was designed as an ultra light weight rail system for the M4 Carbine and all mil-spec AR-15 variants. The SMR MK4 was designed to do a good job in meeting the ten design specifications for a good M4 weapon rail with an emphasis on weight reduction.

Parts used for this Upper Receiver Build:

Bravo Company (BCM) 14.5" Standard Govenment Contour Mid-Length Barrel
Bravo Company (BCM) .750 Gas Block
Mega Arms Forged Upper
Surefire MB556K Brake

Tools needed:

Armorers wrench
Torque wrench
Action Block
Supplied Geissele Wrench
Supplied Allen Wrench

As always, thanks to Stickman for providing the uber clear pictures.

Before starting any installation or work on your AR15 platform, make sure you follow the basic safety rules and clear your weapon of any magazine and ammunition. Double check to ensure your weapon is safe.

First you want to remove your existing flash hider, handguards, and front sight/gas block and tube. You will need your standard armorer wrench, a vise and action block to complete this. The supplied Geissele wrench can also be used to remove the stock barrel nut. Or if you're building a new upper your layout should look like this:

Once you insert the barrel into the upper, you will want to liberally coat the upper receiver threads and the outer face of the barrel extension with anti-seize or grease. In this case, I used moly-grease.

Slide the supplied Geissele barrel nut over the barrel and hand tighten for now.

Use the supplied Geissele wrench to tighten the barrel nut to 40ft-lbs by using the pin and corresponding hole in the barrel nut as shown in the picture. Geissele as well as myself recommend using a torque wrench to torque the barrel nut. However, Geissele notes that if you are an experienced AR builder and sure of your ability to properly tighten the barrel nut, you can do so with a ratchet or breaker bar in the 1/2 square aperture in the Geissele wrench.

Install the gas block and tube. Make sure the tube lines up with the center of the gas tube hole.

Slide the handguard over and onto the barrel nut until it is against the barrel nut. Then use your fingers to align the other end of the handguard. By wiggling the handguard slightly and pressing downward it will start on the barrel nut. Do not force the handguard onto the barrel nut. As you wiggle it on, the alignment maybe slightly off. Just keep wiggling until it is aligned.

Install the 2 rail mounting bolts. You may have to slide the handguard slightly to line up the holes. Do not tighten the bolts fully yet. This is a good time to make sure and check the top of the rails align with the upper receiver. Geissele recommends using a scope mount to bridge the two rails and tightening the mount. After this is done, torque the mounting bolts to 5ft-lbs. That's also engraved right on the rail, in case you forget.

Tighten the 2 smaller nylon tipped set screws against the upper receiver using the 5/64 allen key. The screws are just another extra method of ensuring the rail section does not rotate. A light torque is all that is required.

Your installation of the rail is now complete. Install or re-install your muzzle device, charging handle and Bolt Carrier Group