The problem with a faster twist rate in .223 with the lighter bullets would be the possibility of bullet blowup from overspin. It is doubtful that you would experience this in the .223 especially with 1in9 twist. This is probably the best twist rate for this cartridge, allowing the use of 45-64 grain bullets. You'll have to shoot it and see but I'm betting anything from 45-64 grains will work. Of course, there are other factors involved like the condition of the bore, lighter/heavier loads, etc.
What are you wanting to do with the rifle? Targets, plinking, varminting?
Will you be loacding your own ammo, or just shooting factory ammo? I bet that 1-9 twist will shoot milsurp 55gn ammo really good.
