
Originally Posted by
kwilkin
Personally, my view is that the only thing "Mk18" on the DD offerings is the rail system.* Now that that's out of the way....
Overall, I thing you would be better served with a Colt, BCM, or LMT SBR upper. The DD 10.3" barrel is now well-understood to be over-gassed for NATO-spec ammo. The gas port has been shown to be nearly .1" larger than spec. DD does this so that their guns will cycle with a wider range of commercial ammo. Shooting hotter loads in an over-gassed system will put additional stress on other components critical to smooth operation of the system. This means more regular maintenance/replacement of parts and the potential for unexpected failures. Add a suppressor on there, and the problem is exacerbated. This is not to say that there aren't many satisfied users of DD uppers.
If you want a very close clone of an issued Mk18, I suggest picking up either a complete upper or just a MK18BA (barrel assembly) from Specialized Armament Warehouse. These are all Colt Defense parts and will be closer to spec. The downside is obviously cost.
The LMT 10.5" upper is a good starting point for a Mk18 clone. The barrel is 10.5" instead of 10.3" but they will have a more appropriately sized gas port. Another advantage is that LMT 10.5" barrels have a slightly heavier contour under the handguards which adds additional rigidity while having a suppressor mounted which, in theory, should reduce POA/POI issues with a suppressor mounted.
Both the LMT and SAW MK18BA assemblies will require you to remove the front sight base and install a Mk12/Crane-style/low-profile gas block to install the RIS II rail, so that is something to consider. Also, DD has temporarily suspended commercial sales of the RIS II rails (FDE) while they fulfill a military contract. So, the only way you are going to find that rail is on their complete uppers or rifles. I'm sure they will offer them again, no one knows when that will be, however.
SureFire SOCOM and KAC NT4-QDSS suppressors are correct (i.e., issued) for the M4/M4A1/Mk18/CQBR and will both be suitable/warrantied for use on 10.3" barrels and will clear the rail. AAC suppressors, as is my understanding, need additional clearance on a 10.3" barrel with the RIS II, so a 10.5" barrel is recommended for those. I'm not sure about other suppressors as I'm a clone guy and try to stick to spec.
There is some debate about the optimal SBR length. Some people like to go short as possible, while there is evidence that supports 11.5" as a more optimal length. So, you have to balance how accurate you want to be with parts availability, cost, and which suppressor you plan to use.
*Note also that the "Mk18" is a specific military designation for a particular type of weapon. What DD is offering is actually a clone of M4A1 Block II CQBR (close quarters battle receiver). DD has, in my opinion, hijacked these military designations for marketing purposes to sell to the uninitiated. For example, their "Mk12" is anything but with the exception of 18" barrel profiled for an Ops-Inc suppressor and, like the "Mk18," their M4A1 doesn't have the proper RO921HB-profile barrel. I feel compelled to point this out, although most people don't seem to care about such issues.
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