Many experts will research and experiment with making their own ammunition, hand-tailoring each load with great care until it performs (in their gun) better than any other ammunition in the world. Less adventurous types will consult a "reloader's manual", which contains "recipies" for different types of ammunition. To follow the recipe, a reloader simply acquires the same components and uses them in the same proportions as in the manual. Substitutions between bullets of the same weight may occur during this process, but at any change, the reloader is expected to "back down" the powder charge first, to make sure it's safe, before proceeding to run his normal load with the new bullet.

Understanding Load Data

The significance of these terms:

• Bullet: It is important to know exactly what type of bullet is being used in any given load. A bullet with a cannelure may 'grip' the case much tighter than one without, for instance. And the simple substitution of a heavier bullet for a lighter one will always increase the chamber pressure and kinetic energy being transmitted back through the system. Most load data will not only disclose the weight of the bullet, but its exact make and model.
• Seating depth, or OAL: This is a piece of information present on almost all reloading data, but not present on my 5.7x28mm load. I will say that on mine, I simply replicated the overall length of a blue-tipped factory-loaded 5.7x28mm round which used a 40gr bullet much like the one I was loading. However, in any load data you use, you should look for an overall length (usually expressed in inches, from the very bottom of the rim to the tip of the projectile). If you seat a bullet too deeply in the case, you run the risk of compressing the powder and bringing the chamber pressure up dramatically. To reinforce a previous point, even amongst bullets of the same weight, sometimes their overall lengths are different. It is important to use bullets which are exactly the same as what your load data calls for!
• Powder type: Anyone who tries to tell you that all powders are interchangeable is a fool. Powders are differentiated by their physical structure, as well as a number of factors not readily visible to the reloader. It is worth noting that powders may be composed sticks, balls, flakes, or any number of different shapes. The granules themselves vary in weight and grain size. The surface area and mobility affect ignition speed, burn rate, and potentially other factors as well.
• Powder Weight: Also called powder charge, this is the amount of powder which should be put into the case. Whenever attempting to use a recipe pulled off of a book, or the internet, one should always first try loading 10% less than the weight listed and test-fire that, before moving up to the full stated load. This is known as "working up to" a load.
• Primer: This is the type of primer used in the case. Most of the time, the type of primer to use will be readily apparent by the round - 223, for instance, only takes small sized primers, and it definitely is not a rifle round. Other rounds, such as the 5.7x28mm, are less obvious to the beginner because they're both a pistol and a rifle round. CCI and Winchester primers are largely interchangeable, unless something more exotic (#300 or whatnot) is involved. There are also magnum primers. Selection of primer type can actually make a difference to the amount of pressure exerted on the inside of the case, so it's best to follow your load data without getting "creative".
• Notes: Just about any load data you encounter on the internet, will have some sort of notes attached. In my example, I recommend this load for the PS90 rifle, but not for the FN 5.7 pistol. It may work in the 5.7 for all I know, but I've heard they are far more sensitive to reload variances than the PS90 rifle, so I don't recommend trying it in the pistol.