Can someone or two explain the reason you would chose a certain “GRAIN” of bullet over another. And what grain you prefer? I currently own several firearms (Glock 23, Sig 2022, DPMS 5.56/.223, XD 40 and an old Squire 22 rifle). Up until I purchased my AR about 2 months ago it really didn’t pay attention to the grain of bullet I was purchasing.
Okay this got very long. Quick scan the bolded parts if you're in a hurry.
The short answer is ballistics.
Of course, the science of ballistics is something in which you can practically get a university degree and still not really cover it very well. Which is to say, I'm no expert and what I'll post here are extraordinarily basic. :-)
In-flight ballistics is about how a bullet performs in flight between the time it exits the muzzle of the barrel and the time it impacts the target. Terminal ballistics is the science of what happens to the bullet at impact until it stops.
"Grains" is a measure of the weight of the bullet.
There are 7000 grains in a pound, not that that is necessary unless you're reloading and casting your own and need to calculate material requirements.
There is no such thing as the ballistics of a bullet in and of itself. All other factors of the shot have to be taken into effect in order to be able to determine what is about to happen. The twist rate of the barrel has a lot to do with the flight of the bullet.
A heavier bullet will perform differently than a lighter bullet. Indeed, a bullet that is spinning too fast for it's weight can actually disassemble during flight! Ergo, use the right bullet weight for your twist rate. You'll need to let us know the twist rate on your AR in order to give you any recommendations.
Velocity has a lot to do with ballistics. Heavier bullets are generally loaded "hotter" and will travel faster but it may or may not be a benefit.
Other variables have an effect as well.
As the distance to the target increases, various variables which seem minute will have a more profound effect on where the bullet impacts the target.
On a pistol range at 21 feet, even a fairly fast breeze will not have a huge effect on the in-flight ballistics of the bullet but at 1,000 yards, a slight breeze can knock you off target quite significantly. So can the temperature, the variance between the temperate at which the cartridge was loaded vs. the temperature where you're shooting, whether or not you're shooting downhill or uphill, whether or not your barrel is still hot from the last shot, still dirty from the last shot. Even the coriolis effect of the Earth's rotation can have an effect on very long shots.
Okay, more than you wanted to know, I'm sure. All of the above relates, of course, to in-flight ballistics and again, it's definitely not complete.
Terminal ballistics, as mentioned, relates to what happens to the bullet as it hits the target and travels through it or stops within it. In recent years, bullets have been designed with an increasing focus on this area of the science.
In other words, bullets are increasingly "purpose built." Get the ones that suit your requirements, considering purpose for the bullet, gun to use it in, size of bullet, sometimes even the climate and weather conditions.
If you're buying bullets for hunting purposes, the terminal ballistics aspect is critical for you.
If you shoot a bullet into an animal and the bullet is ball ammo, it's just going to go right through and do very little critical damage and the animal will have a high risk of running away injured but very much alive. On the other hand, if you use an expanding bullet to shoot a bear in the head, it's more likely to bounce off his skull and piss him off and if he leaves you alone afterward, you're lucky. If he doesn't you're not. Ball ammo in this case might be far superior as it can penetrate bone and skull more effectively.
FBI and police agencies put some real effort into making sure their ammo is not going to over penetrate bad guys and continue in a flight patter that is a risk to innocent bystanders. The idea is to make sure the bullets stop inside the bad guys to eliminate or minimize the over penetration risk.
You should consider such questions when buying your own self-defense ammo.
Again, very basic.