Most common 223 / 5.56 ammo is 55 grain, although I see more 62 grain Russian ammo coming for sale these days.

 

However, since I'm reloading, I have a lot of different weight bullets to choose from. What's the relationship between twist, barrel length and bullet weight?

 

My Colt Match Target HBAR (MT6700C) has a 20" barrel with a RH 1:9 twist.

 

For 100 yard competition target shooting, do I want a heavier or lighter bullet than 55gr?

You can range between 45 and 62 gr with that twist. Check this out http://www.sierrabullets.com/index.cfm?section=bullets&page=bc&stock_num=1410&bullettype=0

 

I have good luck with Sierra matchkings for 308.

 

I would say at 100 yds the 52gr will be fine. But I do not shoot competition.

I prefer the Sierra 69 grain BTHP bullet.

I'm actually going the opposite way. 52 grain Matchking bullet, but with a little more powder. No need for the heavier bullet at 100 yards and a 1:9" twist...