I understand one prefers heavier .556? Can somone explain which it is and why?

AR-15 » Ammunition » AR15 Ammo Forum FAQ » WHAT BULLET WEIGHT CAN I SHOOT IN MY <1/9, 1/8, 1/7, etc.> RIFLE?

Heavier bullets are harder to stabilize thus, they require more spin or they wll begin to tumble (kinda good when talking about terminal rate, but really bad for accuracy). When talking about twist the numbers denote,  # of spins/inches. So a 1/7 twist barrel will make a single full rotation every 7 inches. A 1/9 makes a full rotation every 9 inches.

So if you're looking to shoot loads over 55gr it would probably be wise to get a 1/7.

*A side note, some shooters say that having a 1/7 twist being used with a lighter load would have adverse reactions, but I have no experienced this nor theoretically can I accept this. But I'm sure someone can come in here and clear that one up for both of us.

Wrong forum and dupe - please read the FAQ linked to above.