Hi!
I'm working on a custom CNC adaptor to fit a Zentico PT-3 to my G&G RK74 CQB.
I've got through a few design iterations to try and find the neatest solution; initially I was trying for a 'drop in' replacement with a view of keeping the gun stock and the part being able to be bought...
I had this issue when installing a DD rail on a G&P SAI receiver, the solution for me was to send the parts off to Future Tech who I believe shortened the thread on the receiver to allow the rail to butt right up :)
If you haven't already decided yet, I have an article on my little blog discussing my journey into building a HPA VSR which ultimately resulted in me going down the route of 3D printing a custom VSR stock which houses a HPA tank and reg, take a read, it might help...