Read this advice at least TWICE! It si spot on! Al powders have a "burn rate" which tells you how "fast" the powder burns. Higher burn rate are mostly pistols, slower are usually rifles (mix then up and you have a MAJOR problem). Middle is looking at burn rates of the powders and then go for the one in the middle. The manuals get you at a safe starting point. Rifling Twist Rate By Chuck Hawks Inside of a rifle barrel there are spiral grooves, called rifling. These are intended to spin the bullet to keep it stable (point on), without wobbling or tumbling, during its flight to the target. The tighter the spiral grooves, the faster the bullet spins. The tightness of the spiral is called the "twist rate." The rate of twist is expressed as one turn in so many inches (i.e. 1 in 10" or 1:10). The caliber, length, shape and velocity of a bullet determine its optimum twist rate. The standard twist for a rifle barrel is designed to stabilize the range of bullets and velocities normally associates with that particular cartridge out to very long range. Spinning a bullet markedly too slow or too fast is detrimental to accuracy. It takes less twist to stabilize a given bullet at high velocity than at low velocity. At the same velocity in the same caliber, longer bullets require faster twist rates than shorter bullets. A faster twist increases pressure, barrel wear and also the strain on the bullet jacket, which can actually come apart if spun too fast. This particularly applies to frangible varmint bullets, which have very thin jackets, fired at high velocity in very fast twist barrels, such as the 1 in 7" twist barrels supplied on many .223/5.56mn AR-15 type rifles.