The M855/SS109 62 grain load for the 5.56x45 was designed to meet a Nato specification calling for the ability to defeat a Nato standard steel pot (helmet) at 600 meters, which the older 55 grain M193 load was unable to accomplish. A 1:9 twist bbl will stabilize the bullet just fine, the reason for the 1:7 twist was to adequately stabilize the M856 long trace load (64 grains IIRC and a much longer slug).Let's take a 580 series Ranch rifle, with an 18.5", 1:9" twist rate barrel. I know that 55 gr is optimal for this configuration, but what's lost when a heavier bullet, like a 62 gr green tip is fired through it? Does accuracy suffer, and if so, at what range would that become evident?
Lot of ink spilled on the M193 vs M855, but what it comes down to is that the M855 is the better long range loading. Initial velocity is lower, but the better ballistic coefficient more than makes up for this. Basically it carries better. Heavier bullets than this will work even better at long ranges but your tube has to have a fast enough twist to stabilize them.
There have been accuracy issues with some lots of M855, due to the composite construction of the bullet - it has a steel tip and lead core so if concentricity in a particular slug is imperfect the result is compromised accuracy. There have also been reported problems with the M855 due to the lower initial velocity when used with the 14.5" bbl of an M4. If the velocity drops below something like 2700fps the slug tends not to destabilize in soft tissue resulting in "through and through" wounds. At least that's my second hand understanding of the issue.
So, which is the long range load? M855. I would suggest use M193 for anything to 300 yds and after that would suggest use of the M855, which is better for cost considerations in that the M855 is significantly more pricey and less available now.