What is confusing you?
The way I see it, we can provide answers to a specific question you have, limited and worded in such a way as not to confuse you, or we can teach you to fish and eliminate the confusion.
Twist rate is simply how fast the rifling in the barrel turns. It is usually worded as a ratio, such as 1:7 or 1:12. The lower the number, the faster the twist rate, because it denotes the length of barrel it takes for the rifling to complete a full twist. The 1:7 barrel, therefore, has a faster twist than the 1:12 barrel because it completes a full revolution of rifling every 7 inches, as opposed to every 12 inches. Faster twist rates are needed to stabilize longer projectiles. While not technically correct, some say faster twist rates are needed to stabilize heavier projectiles. In fact, it has to do with projectile length more than mass--it just so happens that heavier projectiles in a given caliber are usually longer than lighter ones.
And a grain is a unit of weight (not mass) equal to 1/7000th of a pound. There are 437.5 grains in an ounce. Just remember a grain is really small, and not to be confused with a gram which, is a metric unit of mass. Grain is properly abbreviated "gr." Gram is properly abbreviated "g."
In the 5.56, lighter weight bullets from 40 to 55 grains are usually reserved for varmint hunting. Most are not suitable for self-defense as they are far to frangible, and not likely to penetrate deep enough to reach the vitals of a human assailant. The exception would be some 55 gr loads, such as the M193 FMJ stuff available from Federal and Winchester, marketed as copies of the original military load for the cartridge. While not ideal, it is certainly adequate for self-defense. And I believe Federal and some other manufactures market some bonded 55 gr JSPs for defensive use as well. Generally, however, for defensive purposes, you are better suited to heavier for caliber bullet weights from 62 to 77 grains. This would include the Hornady TAP stuff already mentioned, as well as OTM and HPBT loads from Black Hills, Federal, Hornady, and others loaded with bullets such as the Sierra Matchking. These projectiles are long enough to require twist rates faster than 1:9, but they are known to exhibit terminal performance much better suited for defensive purposes than lighter loads. They penetrate deep enough to reach the vitals, while still fragmenting reliably.
