I have meant to write this for a long time but I have not had the chance to install the DD RIS2.
First of all, the RIS2 was originally one of those Sopmod Block 2 upgrade items - whatever it means, it was intended to be a replacment for the KAC RAS on the M4 for certain occupations . The competition ended up with 3 companies - ARMS, KAC and DD. DD ended up being the winner. All these happened a couple years ago.
The original KAC entry for the RIS2 was the 7" URX. There was also a rifle length URX as well but these days we only see the midlength URX2. So for comparison, I am pitching the original URX against the original DD RIS 2 configuration.
Architecture:
URX attached to the reciver in a manner that is the opposite to the traditional AR barrel nut. In a tradition barrel nut setup, the steel barrel nut fits over the aluminum extension. With URX, the rail itself fits over the barrel extension. A steel barrel nut fits inside the rail and works as a plug to prevent the barrel from falling out. Because aluminum expands more than steel as temperature goes up, loctite must be used on the steel barrel nut. To take off an URX, a heat gun must be used. Also, loctite must be applied between the reciever and the rail - there was one time I forgot to add loctite, the rifle could not shoot for s**t.
DD is rather simple. The barrel nut is no different from the traditional barrel in the way it fits onto the barrel extension. It depends on the tension exerted by 6 steel bolts to hold the rail and the base plate behind the barrel nut together. The advantage of this system is the abscense of loctite and that, if a rail is damaged, the armour can remove the handguard without taking off the barrel nut. If it is needed, it could be done in the field using an optic mount to align the receiver with the rail. If an URX is damaged, it needs to be taken apart completely at a depot.
Installion:
After doing 6 or 7 URX and two DD RIS2, I must say that the KAC URX is much faster to install but it is also easier to screw up. The original URX uses an extremely thin steel barrel nut which could be easily stripped if attention is not given. Also, URX needs loctitie - it is a must. The original GL compatible barrel nut also needs a specical tube wrench to install. Because the wrench fits over the barrel, it is diffcult(but not impossible) to fit a low profile gas block under the handguard.
DD's installation takes a bit more time but it does not need any loctite. it can easily accommodate a low profile/under handguard gas block. On the other hand, installing the six steel bolts could be time consuming if there is not a long skinny tool handle around.
Rail Design:
URX is designed to accommodate the legacy M203 mount as well as a proprietary FF GL mount. Hence, it comes with an ultra thin steel barrel nut so the M203 barrel sleeve can sit on it. Because the rail itself fits onto the receiver, there are more materials surrounding the barrel exension.
DD rail has some pretty complicated contour near the barrel extension, and therefore it has less material. Because DD allows the legacy M203 to be free floated, it does not need 2 channels on both sides of the rail for an addition M203 mount. As a result, DD can make the wall of the rail much thinner. However, DD has to leave some materials at the end of the rail to maintain integrity. As a result, the barrel end of the rail is close ended. One cannot install certain accessories such as Tango grip if the lower rail is attached. It must be detached first. Also, this enclosure eliminates the installation of any other GL such as our own M203A1.
Lower Rail Attachment:
The final version of URX has the bottom rail held on to the main body with a detent. In reality, I had a few occasions when the lower rail came off during firing, if the side screws were not installed. Despite of the washers on these brass screws, they always loosen up on their own and eventually come off if they are not loctited. I cannot imagine depending on 4 of these screws to keep the free float GL mount on the rail.
DD has done a much better job with the lower rail attachment. It is pretty simple - it is held on by 4 machined steel screws with plastic bushing. The good news - they do not come off like the URX's.
Workmanship:
DD has overall better finishing and workmanship than KAC. During installation, we found that KAC rails are generally not as consistent as DD rails. This is apparent when the rail and receiver are kinda stuck in the rail alignment fixture. This problem is especially bad with the rifle length URX from the early days.
URX:
Pros:
Lighter
More streamlined
Potentially more flexible with different types of GL
Cons:
Small details are not well done ( such as the side screws)
installation tool is clumsy and limiting
DD RIS2
Pros:
Easier install and rapid replacement
Less additional cost for ff mouting the original GL M203
Good workmanship and details
Cons:
unable to mount accessories that need to be slided down through the front of the lower rail
may not be friendly with any other GL
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