View Full Version : Centurion Arms CMR Rail Installation

9 May 2013, 01:02
The Centurion Modular Rail (CMR) (http://centurionarms.net/) will be released soon, possibly the week of May 13th. For those AR veterans that are curious to see how the rail is installed, and to assist the novice user this guide is for you.

For this install, I will use a standard M4 Upper with a 16" Centurion Arms Barrel (http://centurionarms.net/index.php?option=com_virtuemart&Itemid=136) with a BCM low profile gas block. I will also utilize Stickman for pictures, as there are enough terrible iPhone pictures on the web already. We start off with a 14" CMR rail. These rails are a different design from the C4 rail as it is a 1 piece design and utilizes a proprietary barrel nut instead of using a stock barrel nut. QD sockets are located on the front and rear of the rail on both sides.

http://i1016.photobucket.com/albums/af282/richdkim77/CMR6_zps25dcc704.jpg (http://s1016.photobucket.com/user/richdkim77/media/CMR6_zps25dcc704.jpg.html)

A close up of the barrel nut and the 3 screws to secure the rail. If you're breaking down a previous build, you will of course need to remove your muzzle device, gas block or front sight post.

http://i1016.photobucket.com/albums/af282/richdkim77/CMR4_zpsf8186532.jpg (http://s1016.photobucket.com/user/richdkim77/media/CMR4_zpsf8186532.jpg.html)

Once you get your old standard barrel nut off along with the muzzle device, gas block/front sight post, the new barrel nut threads onto the upper receiver. As you can see the barrel nut does not have the "teeth" a standard barrel nut has or a hole the gas tube fits through like with many other designs. The barrel nut is very low profile, thus does not have to be "timed" to secure the rail, or have the gas tube fit. This is a design I'd like to see other manufacturers use.

http://i1016.photobucket.com/albums/af282/richdkim77/CMR5_zps1c29e43b.jpg (http://s1016.photobucket.com/user/richdkim77/media/CMR5_zps1c29e43b.jpg.html)

The barrel nut is circular with no flat spots for your standard wrenches. The preproduction model we received did not have instructions, but we decided to use the end our armorer's wrench. A spanner wrench could also be used. I set the torque wrench to 30ft-lbs and secured the barrel nut.

http://i1016.photobucket.com/albums/af282/richdkim77/CMR3_zps9d6e6c10.jpg (http://s1016.photobucket.com/user/richdkim77/media/CMR3_zps9d6e6c10.jpg.html)

Once the barrel nut was secure, we placed the low profile gas block and gas tube back. Then the actual CMR rail was placed over the barrel and secured to the barrel nut.

http://i1016.photobucket.com/albums/af282/richdkim77/CMR2_zps58acc73e.jpg (http://s1016.photobucket.com/user/richdkim77/media/CMR2_zps58acc73e.jpg.html)
http://i1016.photobucket.com/albums/af282/richdkim77/CMR_zpsd4d35afe.jpg (http://s1016.photobucket.com/user/richdkim77/media/CMR_zpsd4d35afe.jpg.html)

The rails were then secured with the 3 supplied screws. 1 on the ejection side, and 2 on the opposite side. The screw holes are recessed to assist you in determining what screws go on which side. Don't try to screw them on the other way... don't ask me how I know this.

http://i1016.photobucket.com/albums/af282/richdkim77/CMR1_zpse24b9662.jpg (http://s1016.photobucket.com/user/richdkim77/media/CMR1_zpse24b9662.jpg.html)

Once you install your muzzle device, you are done. Probably one of the easiest rail installs you will ever do. Hopefully Rainier Arms (https://www.rainierarms.com/?page=shop%2Fmanufacturer&manufacturer=93) will have these rails in stock soon.