Re: Thinkin about an new .223
I like the Vortex Viper HS-T 4-16x great glass, good performance and about $560 at Midway. I realize this is a good bit higher price than you are looking for but if you have some flexibility on budget, then the scopes you find in the $500 range will not be regretted!
That said, if your $300 cap is firm, then some points I would look for:
1. a 30mm tube will give you more adjustment for long range shooting.
2. good glass is more important than objective diameter for light gathering.
3. skip the illuminated reticles in this price range- you will often have to sacrifice more useful features for this option.
4. check the weight and dimensions of the scope, Millet's TRS-1 ($325) for example is 32oz. (heavy) and very large.
Good luck and visit a store where you can look through some options!