John after the rail flex tests that were done a few years ago, I won't span anything onto the rail. Though a good riser like you're using would help.
I'm not particualrly gentle with my guns and have never bent a LaRue mount. I have bent a scope though. I also like the QD mounts and practice with irons pretty often. Best part is we have lot's of great choices now, I can remember when B-square and A.R.M.S. #5(?) were the only options.
My OPONION is to span the upper rail. That setup has proven to be the most stable under hard use in my testing.
The Cantilevers will bend under impact just like a scope tube will without the rings spaced to offer the most support.
Simply put there is no way a Cantilever mount can support the optic as well as a full blown mounting system. If your float tube is not stable then that is a different issue and needs to be addressed.
Lots of float tubes are stable and the fact that a very large percentage of our SOF troops rely on DBALs mounted on the tube in combat shows that it is very possible to have the tube remain stable. Add a riser that bridges the upper/ tube interface and you have a very stable optics mounting platform.
