The powder that you put in the case is the fuel that propels the bullet out the barrel. As such it requires some thought before selecting the correct powder for your application. When referring to loading ammo powder weight is measured in grains. So a grain is a unit of measure not one piece of powder. It takes 7,000 grains to make a LB. so a grain might not sound like much but lets put it in prospective. If your reloading manual indicates that 5 grains is max, then what harm will 1 grain extra do? If you add 1 grain to that 5 then you have a 20% over charge which may very well destroy your gun. Those manuals are printed for a reason and if you try and become cute that can cause you some grief.
There are three types of powders available to the reloader. They are ball, flake and extruded. Ball powder has an advantage in that it meters very well and can be used in handgun, shotgun and rifle ammo. Flake powders are generally used in handguns and shotguns. They can also be used in some rifle applications. It meters reasonably well and if you keep your measure nearly full and tap the side occasionally it helps in the consistency. Extruded powders are generally for rifles though there are a few for handguns and shotguns. Sometimes the coarser ones will bridge in a measure. That means that one load may have very little powder while the next one may have a double charge though that would probably overfill the case and spill on the table. That is one of the many reasons to pay strict attention to what you are doing.
Proper storage of powder is important. It can go bad after some years especially if stored where temperatures are excessive. It should be stored in a cool dry place away from any heat source. If you buy old powder at a yard sale be very cautious before using it. If you see a red dust scrap it. A sour smell might be another indication of defective powder. If you use it that way results can be unpredictable and dangerous. Your best bet is to dispose of it in a safe manner. Fresh powder is cheaper then gun and body parts. Modern powders are not meant to explode though under certain conditions that can happen, usually with severe consequences. When you fire a gun the powder burns at a rapid rate rather then explode. The rate is controlled by various factors such as the coating and size of the grains. There are various mixtures employed by powder makers to control the burn rate. That is why there are so many powders available and it is imperative that you select the correct ones for your particular application. For instance if you take a 30-06 case and fill it with pistol powder and shoot it disaster is imminent. The handgun powder which is way too fast for a rifle case so it will burn so quickly that the bullet won’t have a chance to move down the barrel, hence a blowup. Be sure you read the label on the container and don’t put powders in a incorrectly labeled can. Do not mix powders as that is a recipe for disaster. There would be no way to predict what will happen but it won’t be good.
When you select a powder for reloading a certain cartridge always go to one of the reloading manuals. You should have at least one, preferably more. It will let you know how much and what kind to use. Do not exceed the max loads. Most manuals list from 10 to 15 selections for each load. The reloading manuals are available at any gun store plus online such as Amazon and Barnes. All of your bullet and powder makers produce them and they are all good. Examples are the Hodgdon, Sierra, Speer, Nosler, Barnes and Hornady. It doesn’t hurt to have all of them.