Re: 40gr bullets in 223
That is the nature of the beast (so to speak). I have not seen the 450 fps variation you have, but yes, they do seem to vary more than heavier bullets. I was taught "variables" in your loading can have a more pronounced affect on a lighter bullet. My experience has been, high quality, lapped barrels (.223) are quite consistant, while some factory barrels with 40 gr. bullets show much more spread.
I try hard to get them as consistant as possible, but my 40 gr. loads are rarely used past 350 yds. I can live with it.