From those numbers it looks like the 55 would have the time-of-flight advantage to maybe 300m, and then the 77 would negate the difference.

I wonder if those principles would apply to something like the 4.85x49, if you gave it, say, a 63grain. 

smg762 said:

From those numbers it looks like the 55 would have the time-of-flight advantage to maybe 300m, and then the 77 would negate the difference.

Out to 400 metres all these rounds perform similarly. The M855 has a BC of about .156 and the mod 262 about .2. The current Russian round is the 7N10bis    .176 at 900 mps.  The generally low performance of the 5.56x45 is due to the lack of room for projectiles. Increasing the OAL by 5 mm or using a 40 mm case based on the 6.8 SPC are the solutions to that issue. 

You mean just like 224 Valkyrie?