I know that VLTOR VIS provides continuous system that should flex less then the other two and that DD is supposed to flex less then Troy.
What I do not know is how do they compare based on the quality and weight and how much flex are we really talking about.
It would go best-to-worst in this order.
VLTOR VIS
DD Lite Rail
Troy
Another very rigid option would be a VLTOR MUR receiver with a DD Lite Rail. The billet upper receivers like the VLTOR, LaRue, Sun Devil are more rigid than a standard forged upper receiver.
gotm4,
Please elaborate on differences between the VIS and a conventional receiver/forend configuration in terms of one’s ability to change barrels, etc… Thanks!
Either one uses regular AR barrels.
Forgings are stronger than billets. Because a forging is forged in the shape of the final object, it has much better surface hardness and structural integrity. Even extrusions have more strength than billets.
A billet on the other hand will be much more precise, and as far as I know, should be effected less by heat due to being machined from it’s natural form instead of forced into a new one.
Atleast that’s the way I’ve always understood it. I’m by no means a metallurgist.
To the OP, gotm4 hit the nail on the head with his list. The VIS does set some limitations, but I think it’s probably the most practical modular upper available.
Unless the billets were reinforced (like the MUR and VIS) then I would say that they are stronger than a forging.
The VIS represents one of the finest upper receiver/rail system on the market.
If you can install a one piece FF rail or afford the VIS, you should always go that way for two main reasons (strenght and weight). If you cannot install a FF rail or afford the VIS, then the TROY rails offer and excellent product.
C4
Here’s an article that touches the bases on forging versus other methods. Check out “the downside of CNC machining” section.
http://www.sheldonbrown.com/dp-forging.html
The only strength benefit for billet over a forging is stress in the grain structure, which in the case of a forged receiver will most likely only become an issue when severe heat is involved. In any event, a good heat treat will pretty much remove any possibility of warping or cracking whether it be from stress or heat.
Billet receivers are a good idea, especially with even the best manufacturer’s forgings being flops. In the case of an AR receiver (upper or lower), the strength probably isn’t an issue, and it’s the precision of a billet receiver that makes it all worth it.
I think that would be true if you were comparing apples to apples. Strength and rigidity are two different things. Titanium is very strong but it’s is also pretty flexible (which is why it’s harder to machine than say common steel, titanium has a little ‘give’ to it).
All the billet uppers that I’ve seen (VLTOR, LaRue, Sun Devil etc) are much thicker/reinforced in a lot of areas when compared to regular ‘mil-spec’ forged type uppers. Thicker to make them more rigid than forged uppers for less flex. Less flex = good juju ! 
Comparing a VLTOR upper to a Colt forged is like comparing apples to oranges.
A very good read when you have the time K.L. Davis’ Flex testing
Every time I hear this brought up, I wonder if anyone has looked or found out what the billet comes from. I’m curious to know how many times something is forged until its weakened through its grain, and how many times a billet is forged into its shape. Its my understanding that there is much more to it than simply forged vs billet, and that is after talking to the metal techies.
Exactly, billet is a shape and forging is a process.