I've played around a lot with .223 caliber bullets this past year. So much in fact, that I purchased 4 different powders, 3 different primers and about a dozen different bullets in weights from 45 grains to 75 grains. Powder burn rate and volume certainly have an impact on accuracy but rate of twist REALLY makes a huge difference. The rate of twist of your barrel may not stabilize a 60 grain bullet in your rifle ( I've owned rifles with 1/12, 1/9, and 1/7 twist) Bullet shape, length and weight are all important variables. I would suggest that you select a lighter bullet - maybe 50 grains and see if that makes a difference. Look up "Ladder Method" on the internet and consider loading 20 rounds using this method. I am pretty sure that you will be able to find the perfect load for your rifle. Heck, if you want to send me a shipping address I'll mail you some 50 grain bullets to try.
Good Luck & happy loading!
Paul