Bullet length (and density) determines the rifling twist needed for stabilization. If one is dealing with pure lead (or pure anything) longer also equals heavier.
M856 Tracer is stable with 1/7. I've not seen tumbling tracers, even in cold weather. Something tighter might be more accurate. Tigher twist, however, creates more wear -- no free lunch. Faster twisting motion means more strain and friction. 1/7 is a compromise to produce as close a trajectory match between M855 and M856 as possible. Tracers are suppose to show where Ball is going.
For non-tracer firing 1/9 seems to be the "universal twist" and is kinder on the barrel. Most match shooting seems to be 1/8 twist.
Assuming the bullet is stable in air (no M855 from M16A1 1/12 twist please), the rifling twist has no effect on the bullet's stability once it enters heavy media such as water or animal tissue. Bullet tumbles right now! By the time the bullet has penetrated 4" it's tumbled 90°. 4" distance is a microsecond in time. [I'm sure someone can do the math, I've been killing Sir32 virus all day [;)].
55gr M193 from 1/12 and 1/7 twist barrels produces the same wound profile.
I personally prefer the Vortex supressor to all others. Nice thing about 16" bbls is you're not locked into any particular one. Beware that there are still some bad Vortex out there. Long time member and contributor Dave G. reports in another thread of a bad one. Shoot one on your 16" or 20" and if it survives have it welded on the 14.5" to make it NFA legal.
14.5" bbl is handier than 16" which is handier than 20". Nice thing about these rifles is you don't really have to make a choice. In a matter of minutes I can assemble a semi version of the M16A1, M16A2, or M4 Carbine and still have a working flattop Dissipator with ELCAN as a spare.
-- Chuck