How far out are you trying to reach? As a practical matter, 300 yards is probably the max distance at which a 50ish grain bullet could be considered to be the best thing to use. Beyond that, you'll likely want something like a 69 or 77gr bullet. The lighter bullets that rely more on velocity for their energy will also bleed off more energy down range.
What are you trying to do?
For my fast twist .223s, I like the 77gr SMKs over 24gr of Reloader 15 using a Remington 7.5 primer. It's sort of a "standard" load for a fast twist .223. As for terminal performance, I don't know what to say. The 77gr SMK is NOT a hunting bullet and Sierra tells you that, but they sure did make a mess.
Just keep in mind that many comparisons that you hear may not be fair. In one of the above posts, a gent claimed that the 52gr ballistic tip was more accurate than the SMK. But...that was with the same power. That's not to say that the SMK wouldn't have been more accurate with a different powder. In my limited experience, I've had better luck with ball powders with lighter bullets and stick powder with heavier bullets.
Still, if I wanted to "stock up" to cover all of my .223 needs, I'd opt for the 69gr SMK. This bullet has shot well in all of the .223s that I've tried them in that had 1:10 twist or faster barrels. I even had one .223 with a 1:12 that really liked these. If I knew that I'd only be shooting 1:7 and 1:8 barrels, it would be the 77gr SMK.
As mentioned before, the 75gr AMaxs do shoot well, but they are too long for magazines. Unless you're up for single loading, avoid these.
