I have read, and reread Troy's post on the best loads for .223 weapons. Outstanding I must say.
My question is in the slower twist barrels at what range does the heaver round start to loose stability; 0 to 50 yards 50 to 100 yards or beyond.
If you take a 20in 1/9 twist and try and shoot 75gr how far would you expect the round to stay stable? How much difference would loading to 5.56 rather than .223 make?

As far as the barrel twist-stabilizaton ratio, all barrels are different.  Some people's 1:9 barrels handle 75 or 77 grain ammo well, some don't.  (If you look at a box of Hornady 75gr bullets , not ammo, you will see "for 1:9 twist" on the box)  5.56 powder loads should give you  150 to 200 fps more than .223 load generally speaking, which means longer fragmentation range.


TS

My Bushmaster 1:9 twist 16" Hbar shoots nice 6-8" groups at 300yards with nice clean holes with 75gr Black Hills Blue Box when I am up to the task(which is rare). Even my nasty 12-16"  10 shot groups show no signs of keyholing. I haven't shot paper past 300 yards yet, but I can get consistent hits on "man-sized "gongs at my local range out to 500 yards with the same ammo and a good scope. YMMV