The vast majority of .223 cal rifles nowadays are built with 1:9 twist barrels. 1:7 is considered best for shooting 80-90 gr bullets for match target purposes. The normal twist was originally 1:12 since that was considered best for 50-55 gr bullets in the 60's as translated from performance of other .22 cal centerfire rounds, like the .222 Rem.
The 1:9 was chosen because it stabilizes 55-70 gr bullets well, and is now considered standard for the cartridge. A quick look at the specs for many .223 cal rifles will verify this point.
The most common loads for .223/5.56 now are using either a 55 gr or 62 gr bullet.
Unless you are shooting a gun other than a semi auto for this round I see no reason to shoot anything other than 55gr bullets. My reasoning for this is simple. They are more common and easy to find in bulk than any other size. I just bought 6000 of them from Mid South for $471.68 shipped! that's .07 each for Hornady 55 gr FMJ's and they had the same deal for SP's! Seems they kind of over ran those bullets and over estimated demand.
Previously I have shot recycled or Pulled bullets in my .223's. Once again 55gr FMJ's and the last batch I bought were .09 each! HA the new Hornady's were a better deal and it just shows they were trying to get rid of them as fast as possible. Gotta love blow out sales.
Sidenote: I got my Havalon knife from Midway for $19.95 on a blow out. normally $40+.
I load all my .223's the same way. 25 gr of BLC2/ W748 with a 55 gr bullet with a Lee Factory Crimp on my D550B. This is my standard load for the .223 and is well below max for the powder and the bullet combo. All of my guns feed and shoot it well enough to hit a 12x18 steel target at 300 yards using a Red Dot sight. I consider 300 yards to be the maximum effective range for this cartridge from these guns.
Many other powders work well including H335 and many of the new wonder powders.
The guns are Kel-Tec SU16CA, Bushmaster Carbon 15 AR and a Mini14. None of these guns are tack drivers nor should they be considered as such. They are defense guns and whereas the KT and AR will shoot about 1.5" at 100 yards with a scope mounted, the Mini 14 is closer to 2.5". 2.5" is still 7.5 at 300 yards which is more than adequate for the intended purpose. In my case a man sized steel target. All these guns have TRS 25 Red Dot sights on them for quick target acquisition so accuracy is slightly less, but still adequate. The guns are all sighted dead on at 50/200 yards and are 1" high at 100 and 12" low at 300 yards.
I am very big on finding a good shooting load for every caliber I load and sticking to it. This way I spend more time shooting and less time searching for the magic load that will make all my dreams come true. The whole Idea is to shoot at stuff not waste ammo trying to wring out the last Iota of accuracy from a round that is not meant for that purpose.
The .223/5.56 round was not designed as a Bench Rest round. It was designed to kill people. It works very well for that purpose and has been optimized over the last 50 years to the point where the research portion of the game is pretty much over. It works real well when loaded the way the factories do it.
This is all MHO, but it is based on many years of shooting this round thru many different guns. What I have conveyed in this post works and works well, so if you don't know where to start,,, try it.
Randy