Re: need a 12 twist load for .223
Being a twelve twist... generally I would tell you to load up a 52-53gr SMK (or really a 50-55gr anything) over either 25.0-25.5gr H335 or ~26.5gr Win748, pick a primer, pick a case, and go shoot. If that doesn't drill bug holes, then there is either a gun or a shooter problem, cuz it ain't the load - it works way too well in way too many guns. And then I would tell you to forget about shooting 500-600yds w/ that load - light bullet with little to no BC to speak of is going to get thrown all over the place by even the lightest breeze at those distances. 300, maybe 400, on real calm days.
We have a gentleman who is shooting 500-600yd F-Class matches that I go to, using a Rem700 SPS in .223 w/ a 12 tw barrel. He doesn't do especially well, because of what I mentioned above (wind), but he does have a good time and is learning a thing or two about the wind and his gun. I believe his current load uses Berger 64gr bullets over 26.5-27.0gr Varget - which seems a bit warm, but the 27.0gr load definitely held better elevation on the target @ distance. Even he's realized that he's kind of fighting an uphill battle as such, and has a new faster twist barrel on order. Having seen what a .223 with a *fast* twist (7-8) can do, I think he'll be more than happy when he gets it installed ;)