Every rifle's barrel is designed with a specific purpose in mind. Some are rifled to put the optimal spin on a very heavy bullet, intended to stretch shots so far, you can't hear the steel ring. Others have a slower twist and are more intended for lighter grained, shorter bullets. Rifling is usually written as a ratio. A 1:10" twist means that there is one full revolution of the rifling in 10 inches of barrel length.
You'll find yours in the paperwork that comes with any standard gun, or you can identify your own twist using a cleaning rod. Generally speaking, the longer and heavier a bullet is, the tighter twist it requires to stabilize. If you prefer to shoot lighter, shorter bullets, the tighter twist might cause instability in the projectile, causing your groups to spread and you to have an unhappy range day. The solution? Use this video as a guide for what to start with (for your rifle chambered in .223/5.56). For other calibers, just give us a call or drop us a note. We'd be happy to help you choose the right ammo that meets your shooting goals.