Question in the title. I understand the M856 bullet is quite long. Will a standard 1/9 barrel stabilize it? If not, will 1/8? Or is 1/7 required?

If this is in the wrong place I apologize.

I doubt the 1/9 will stabilize a M856 correctly, but you'll never know unless you try it in yours. Barrels are all a little different. The M856 is as you know a very long projectile and is the reason the military went to a 1/7. I would hazard to guess that a 1/8 has a much better chance with the tracer round.

M856 can be found almost anywhere online. Pick some up and try it. It won't hurt you gun to shoot it. But don't try it through a suppressor. That would be bad.

edit: By "correctly" I am referring to longer distances. At 90 meters or less they should be OK. Anything after that and you might as well just be   throwing a rock.(accuracy wise.) I have no trouble firing a M855 out of a 1/9 but I have never tried the 856.

The 16" velocity should be above 2,800fps break point for the twist calc, which is telling me
1-8.25 or faster for a proper twist.  1-8 and 1-7 will be ok, assuming above frigid outside temps.