Actually, there is only one D-Fender.
There may be some confusion, because our early production models were blue, and all the newer ones are black.
There is no difference in the performance of these, only a color difference.
All the D-Fenders we are shipping now are black.
Regarding your application on a 20" rifle, it will improve your extraction reliability, but will not have as much effect on reliability as it would on a carbine or SBR. The rifles don't suffer as much from the extraction problems, due to the longer gas system length. We sell far more D-Fenders to carbine and SBR users, than to rifle users. However, it will not hurt the performance of a rifle, and may increase reliability in difficult circumstances, such as dirty conditions.