Welcome to a great forum !
I have a D&E gun about 80% complete but the project got stalled when I relocated out of state. I am just now getting close to jumping back into it.
What I learned in the meantime, and wish I knew when I started is the differences in the D&E and RGG designs. The D&E design is loaded with problems which make for a gun that flat out does not function. A very few have come up with patches to get the gun to work to varying degrees. The only plus to the D&E gun that I know of is that it is a near true scale model of the original 1874 gun.
The RGG gun is the one that is a far better design. It works nicely, as proven by the many that have built and fired them. The gun is not a scale copy of the original like the D&E, it was designed for functionality. The RGG departures from a true scale copy are minor and would only be noticed by someone extremely familiar with the originals. Over the years there have been revisions to the RGG drawings as well as minor changes recommended here by members that have built the gun. If you go the RGG route, check here for the latest revisions and suggested departures from those drawings.
Knowing what I know now, I would never recommend anyone to build the D&E gun, the RGG is the way to go. Unfortunately, I am about 80% complete with the D&E so will complete it and hope to get it to function with the patches folks have come up with
George