When a rifle is fired the barrel begins to oscillate or vibrate. Experts agree that to get the most accuracy from a given rifle you want each bullet to exit the barrel at the same point in the oscillation sequence. For our purposes, I call that the sweet spot. In almost every shooting magazine where firearms are evaluated, you will find a table showing brand/type of ammunition, velocity, and accuracy. Different manufacturers load ammunition to slightly different pressures and velocities within the industry limitations. Different powders have different pressure curves causing variations in the vibrations. Bullet weight, shape or construction can also impact this phenomena. Ammunition that exits the barrel closer to the sweet spot, or same point in the barrel oscillation, will be inherently more accurate. Two apparently identical rifles from the same manufacturer may have different ammunition sweet spots. Reloading allows us to adjust or tweak the components of ammunition in search of that sweet spot for your firearm.
The variables you can manipulate are brand of primers, type or weight of bullet, bullet seating depth, powder charge, and type of powder.
Primers from different manufacturers ignite the powder at slightly different rates, changing the pressure curve and bullet velocity.
Popular bullet calibers have hundreds of bullet options in brand, construction, weight and style. Even less popular calibers have a many bullet choices for reloaders. I had a 1903A3 30-06 that was most accurate with 150-168 grain bullets. It didn't like 220 grain bullets at all. I occasionally loaded 90 grain bullets to plink with. They weren't particularly accurate but they were fun to blow up jugs filled with water. You can't change the rifling twist rate in your barrel, but you can choose bullets in a weight range it is optimized for. It has long been known that heavier bullets require faster twist rates to properly stabilize. Heavy for caliber bullets in my old 30-06 would keyhole. That is, they flew like a wobbly football and made oblong or keyholes in the target. A current AR 15 manufacturer lists optimum bullet weights which differ depending on the rifling twist rate. An internet search of rifle twist rates will give you information to select bullets in the weight range optimized for your barrel. I'll write more about this in a later post. Obviously, you want to begin with bullets suitable for your purpose in a weight range compatible with your barrel twist rate.
Reloading manuals give several powder brand/type choices for various bullets. Choose a powder that works well in the weight range of bullets you are interested in. Changing powder charge weights slightly is the cheapest and easiest place to begin your load optimization search. Begin with powder charge weights 10% below maximum. Make 5 bullets at charge weights of 10%, 8%, 6%, 4%, and 2% below maximum. One of those groups should shoot better than the others. That is the place to begin further refinement OR if that is good enough for your purposes, you are done.