I want to do another piston build because why not and am considering using the 6940P as a base carbine as I don't own a Colt yet either.  Here is my main question though:

Anybody know if the piston system will work with the Daniel Defense RIS II FSP if I use another upper receiver as well?  

Any help would be appreciated.

Thanks,

Nik

I'm pretty damn sure the answer is no.

All of the 694X series rifles (including the Piston rifle),use a proprietary barrel nut, and barrel extension (among a few other parts). Designed to work with the monolithic upper it comes with.
Won't work with a standard upper.

I was kind of thinking that might be the case, but technical info on these things seems somewhat difficult to come by.  Thanks for the help.  

Still want a piston Colt though now......