Does anyone know how,heavy 556 ammo compares in terms of flight time, at say, 400m

Like if you compared the 77grain to a 55grain, would the 77 have a faster time to target at rhose 400m ranges?

Obviously the 55 starts faster, but i was wondering how aggressively a 77 would catch up....if at all

For easier comparision, the following assumes the same G7 form factor (1.10) and identical muzzle energy (1620 J) for both bullets.

That means the lighter (3.56 g) projectile will start at 954 m/s, the heavier (5.0 g) at 805 m/s. 

In these circumstances, the velocity crossover (speed about 490 m/s) will happen at a distance of about 465 m. At this point the lighter bullet still has about 26 percent of its muzzle energy, the heavier retains a lot more, 37 percent. Due to losing less velocity, the heavier bullet has a larger kinetic energy over the entire trajectory.  

But the time of flight to 465 m at 0.68 s for the lighter bullet is still faster than for the heavy bullet at 0.74 s. The time of flight crossover distance is at about 800 m after about 1.60 s. Only at this distance the heavier bullet physically overtakes the lighter.        

The crossover distance for wind sensitivity is at 600 m. Beyond this point, the heavier bullet deviation due to constant crosswind is somewhat smaller than for the light bullet, which is better from the muzzle up to 600 m in this regard. 

The above figures are only valid for the assumed muzzle velocities and identical aerodynamic drag (form factor 1.10). In practice, lighter projectiles are usually shorter. This means a less favourable form factor, which for 5.56 mm 55 gr bullets is typically closer to 1.30. 

As you can see, a lot of factors come into play and you need to use a ballistics program to answer questions like the above.   

Thanks. I was surprised that 556 reaches 450m in about 0.7secs....i thought that kind of time was reserved for the 17 rem.

Its often said that 77gr massively outranges 62, and is just generally a win win option. I wonder if this is more to do with retained energy.

I asked the original question because one of my projects is a 4.9mm (thats he actual bullet diameter) doing about 1100lbs at the muzzle.

I assumed that a 48grain would give the best time of flight, and wasnt sure if upping to 55grain would ruin things 

(Obviously downrange energy would be better)

EDIT

I just noticed that the  556 time-of-flight difference is virtually negligible at 450m.......like 0.05secs....am i reading this right?

Yes, you are.

Such negligible differences are exactly what preclude new designs being adopted.

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?