I had a Fobus dual rail on my shorty AR 15 for a while. It locked up nice and TIGHT, but even so I never trusted the top rail for optics. In fact, I didn't really want or think about using the top rail .... all I ever used was the bottom rail to mount a vertical forward grip, and potentially a bipod.
The Fobus dual rail setup was nice ...
light, compact, and it seemed stronger than an add on rail with the stock handguards.
Right now I am running a full floating rifle length alloy quad rail on my shorty. While it does hold a red dot solid way up front, where I like it, I find the quad rail is too fat for comfort unless using only the VFG. I am thinjking about ripping the two side rails off except for a short section at the front.
Of course, YPMMV!
LAZ 1