Originally Posted By: jim87formula1 and 9 twist + heavier bullet = happiness..... There's a lot more factors in there but I would go with a 55gr Vmax and start there and work yourself up if those aren't doing the job...
+1 on Jim's post. The lighter (55gr and below) work best in the 1 in 12 twist barrels, I would start with a 55gr and work up from there and see if it helps any.
If you want to save yourself some work buy a few boxes of factory ammo and see what grain weight of projectile your rifle likes best, then start refining your loads.
I had to try 40-55gr projectiles from the Sierra 40gr HP's, VMaxs, BT's and some old 45gr SP Sierras that my F-I-L had left over from the 80's before I figured out what my rifle liked.
All of the comments regarding starting low on your charges and working up are 100% on the money. Pull out your reloading guide and start at the minimum and go up in .5gr increments until you get to max, then you just need to figure out which one shoots best and go with it. Just FYI speed kills (even with shooting). Not every booming max load (I know..not exactly booming in the 223) will shoot good, most of my loads are around 3000-3150fps with 40 and 50gr nosler BT's. Not terribly fast but they shoot magic in my rifle.
You might also want to check your rifle bedding and make sure your rifle is properly floated and fastened down right. Not all rifles shoot .5in groups just FYI. There is always the option of spending a ton of money on a stock, trigger, barrel and sight that can give you the potential of doing it, but that rifle may not be the one that can. I have one that I love above everything in my safe, an old pre-Garcia Sako L579 in .308, she is a grand old rifle and never has failed to bring in the bacon come hunting season...but the best I have ever got out of her is about .75in. Nothing wrong with that at all though, as many whitetail, hogs and sheep floating around in critter heaven can attest to.