J&T uppers do accept .223 ammo.
The Shaw 24" SS bull barrel is 1x8 rate of twist.
Accuracy recommendation: Control the variables (ammo, bench, respiration, trigger technique, heart rate, wind cycle, concentration), and Practice, Practice, Practice.
Bullet weight is relative to range, conditions, charge, twist, and several other conditions. Look into a good markmanship book, maybe one (or more) of David Tubbs. The man hits at a mile.
J&T has a great new catalog you can get, and a good website which provides a ton of info on their products.
Hope this helps. Don't forget to have fun with your AR.