In my experience, the heavier flatter rounds, like the Mk262 will start to shine at longer distances. You will not likely see much difference with quality ammo at 100 yards. For example, 55 grain, from a reputable provider, like M193 will be flatter and faster at 100 yards, but will start to tumble as it becomes transonic past 300 yards. You might find that Federal 55 gr beats the competition at 100 yards. But it is like throwing a donut at 400 yards.
That is one reason I asked about your barrel. 1:8 at 18" will only make a difference beyond 300 yrds. At 100 yrds, a 14.5" quality barrel will do just fine. At 400 yards, the muzzle velocity is just too slow, and will drop. Take your 18" barrel out to 500 yards, and now you will see results with Mk262, or M855A1. The best ammo at 500 yards, IMHO is the 77 gr TIPPED Black Hills.
Hard to explain all the ballistics in short note, but the issues with 5.56mm are 1- wind drift (lighter weight bullet, very susceptible to wind drift, 2- transonic transition (the bullet starts to tumble and loose focus as it passes the sound barrier), and 3 - bullet velocity, which then affects drop, wind drift and transonic issues. In reality, this weapon performs well at 25 to 250 yrds. After that, everything is a compromise, and engineers have created faster twist barrels, 5R rifling, 18-24" barrels, and 77 grain ammo, all to push this platform and projectile beyond its initial design.
It is a fun gun and a fun round to work with, but it is like DOD originally purchase a VW Bus, and asked it to win the Daytona 500. Alot has changed since introduced into the M16 in the 1960s, but at the end of the day, there are better projectiles, like the 6.5 Grendel that can be reverse engineered into an AR-15, if you really want to hit targets at 600 yards. But, I digress.
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