1/7 twist is more ideal for the heavier stuff. It will put more RPM's on it which comes in handy in freezing cold weather.
MK262 is hot stuff. It isn't dangerous, but it is definately hot. Hot enough to where brass takes a beating and life is shortened. MK262 is something like 2848fps from a 20" barrel for a 77gr bullet. Most saami data is in the 2600fps range. Remember that a round gets even more pressure after cooking in a blazing hot chamber after a few rapid fire magazines.
Any NATO chambered AR-15 should be able to handle it fine. I don't think anyone chambers any barrel for an AR-15 that would be dangerous. The manufacturers and barrel makers automatically protect against much of the danger by making chambers that can safely shoot the heavier bullets at NATO pressures. They look at what's on the market and what people can handload and go from there. There are some extreme exceptions though.
The Mk262 is pretty much the ultimate in 5.56 ammo these days. I also believe it is the pinnacle of the 5.56x45. People have experimented with 80gr, 90gr and even 100gr. They are simply pushing the limits past practicality and are even counter productive in many ways. 5.56 has come a long way. There isn't much left to do in bullet design either. The 69gr and heavier stuff is long - so it has earlier yaw effects. The heavier 5.56 fragments much more as well as more often. People tend to forget that the 55gr and 62gr stuff looks fantastic when they do fragment, but they don't always. The match-grade thin jacket stuff is much more consistent, as well as having more fragmentation effect.
I don't see anything coming out in the future that is magazine-fed that can outperform the Mk262 and being worthwhile. If Mk262 isn't getting the job done, it's time to switch cartridge entirely. This cows been milked.
