I have the dd mk 18 upper. I have had no problems with it. I run a AAC M4-2000 on it 95% of the time. It just looks like the suppressor is "supposed" to be there.
Unsuppressed, however; I REALLY prefer my 11.5" BCM upper.
As a side note, I have had a lot of problems with Geissele triggers (S3G and SD3G) while running these sbr's suppressed. (Failure to reset) Triggers work fine with longer barrels and on the shortys without the can.