Thank you. That's the kind of difference I want to know about. I'm still curious of how much (insert the way to quantify) this affects 6061 rails. And to be clear, how much deflection under "Combat" or "Duty" use is applied to rails. Can a 6061 rail be sufficient to prevent permanent deflection or is 7075 the only way? If you go sledge hammer style on a dude with your rifle, I'd expect stuff to get bent, no question there.
This has been done privately by a number of different organizations but I am unaware of any publicly available studies. It is people that have seen these private studies that are saying Geissele, Hodge, and KAC. I'll also throw in the LMT monolithic uppers even though they technically aren't rails.
What this comes down to in its most simplified form is a single side anchored deflection problem. This can be tested by adding a force at difference lengths along a rail that is attached to a statically anchored upper. The considerations to look at are how far the rail deflects for a given load (important to know for true deflection of a laser from things like slings and bipod being loaded) and how the rail returns to zero (important for retention of laser zero over repeated deflections).
Basically for anyone that doesn't have experience with the four options being discussed, the classic free float rail that we are talking about is broken into two pieces, a rail and a barrel nut. The more contact between these two, the better off we are for being able to get as close as possible to the true material spec for yield because it becomes more and more like a idea single anchor beam deflection problem. The three best solutions to this problem have been the very long and sturdy barrel nuts like from the SMR and WedgeLock/PinchLock family of rails, making the rail and barrel nut one piece like the Knights rail, and making the rail part of the upper receiver like the LMT MRP uppers. Once you have a solid barrel nut lock up you begin to look at both the geometry and material of the rail in regards to resisting and returning from deflection. This is where 7075-T6 has a significant advantage over 6061.
As far as how much this effects the real world. You can see deflection down range when loading a bipod or a sling hard. This is why you want to have the RAPTAR mounted up by the scope and not on the rail and why people will move their sling back by the receiver when using an IR laser. You can also move the IR laser further back towards the receiver to minimize the effects of deflection.
Can 6061 be sufficent? Absolutely, with enough material and a short enough rail. There are also many other considerations in a upper receiver package like the inner diameter of the rail and the length of the gas system and gas block design which, if not properly clearanced can hit the rail and can cause forces to be applied on the inside of the rail. This is the issue that has been talked about with the MK16 and the URG-I which I believe the original post is alluding to.