First and foremost
WELCOME TO TFL!
There are many powders which will give excellent accuracy in the 223. Your weapon will have it’s own likes and dislikes. And may like entirely different powder/bullet combinations than an identical rifle. Bottom line - - - you will just have to test several.
The twist of your barrel will dictate the use of certain weight bullets. You should know what your twist rate is before going further.
My recommendations for a 55 gr bullet would be to try Varget and TAC as starters. If neither works well, try BL-C2 and either IMR 4895 or H4895. I have not tried the newer powders Benchmark or IMR 8208 XBR .
Below is a listing of a few factors I have come across which affect accuracy. Other folks have published articles and books on these categories.
Primer: strength, brisance - is a measure of the rapidity with which an explosive develops its maximum pressure
Barrel: length; tightness of bore; height of the lands; distance of bullet to lands; temperature of barrel;
Lock Strength: the force which is applied to the primer by various strength firing mechanisms;
Bullet: bearing surface of bullet, alloy of bullet; shape of bullet; distance off rifling;
Brass: new/used elasticity; manufacturer, volume; crimp;
Powder: new, aged, old, batch powder was from;
Weather: ambient air temp., barometric pressure, humidity
Elevation: above sea level
Other: I am sure I have not listed all