There is no replacement for practice when it comes to developing skills with your firearm. As well when you practice you can determine the attributes of your gun. Like all mechanical objects there is some variation from item to item, and the sooner you discover the qualities of your particular one the better off you will be. Only through practice will you found out what a given firearm and ammo combination is capable of in your hands.
For instance, I know from practice that I can't take a standing shot at a deer that's more than 75 yards away and ensure it's a killing shot. Only by doing this in practice did I ascertain my limits. The ammo and certainly the rifle I use are more accurate than that, but I can't keep every shot inside a 6" circle when standing at a distance any further than that. I can keep most of my shots inside 6" at longer distances, but for hunting standards I will only take a shot if I know it's an ethical shot, not a wounding one.
For handguns do you know your handgun will work properly with a given type of ammo? You can only find this out through practice. I have a gun that came with an MSRP near a grand (not that I paid anywhere near that!) that didn't work with a couple types of ammo. From the factory it was equipped with a spring that worked best with hotter ammo, and some of the slower rounds wouldn't cycle it all the time. Here's another gentleman that had a similar issue with a gun and ammo combination. There is no way to find that out other than to practice.
Pairing a rifle with ammo it prefers (for lack of a better word) is largely trial and error. If you make your own ammo you can take measurements to help arrive at the best choice, but if you rely on store-bought ammo (like most people) you will likely have to try a couple different types or brands of ammo to find the one that works best in your rifle. In today's age where information abounds and is easy to find on the internet it's tempting to skip this practical exercise. You may have read that someone else achieved great results using gun X when they used ammo Y. You may think to yourself "I've got a gun X as well, so I'll just buy ammo Y and be done with it.".
In reality, while ammo Y would be quite excellent, it may not be the best choice for your particular gun. How your chamber is cut, your barrel length and the type of rifling in your barrel all matter when matching it with ammunition. The only way to find that out is to try. Starting with ammo that other people have had good luck with is a good start, but even if you both have the same model of gun with the same barrel length chambered in the same cartridge there are going to be differences.
The machines that make the gun, cut the chambers and rifle the barrels all have tolerances. When they reach the edge of the tolerance levels the cutting heads are replaced. A gun made when a cutting head was new is going to be slightly different than when one was on the verge of being worn out. Both are going to be perfectly safe and within specifications, but it may affect what sort of ammo each prefers. You can only find this out by practice time trying different cartridges.
Hunters have other considerations as well, and that is trajectory. I recently went to the range with two AR-15's with different barrel lengths. The resulting difference in velocity changed the trajectory of the ammunition I was using that day. When both guns were zeroed (sighted in) at 200 yards, the one was about 2" high at 100 yards. The other one was 3" high. That's not a big difference, but it goes to show how the same cartridge will have a different trajectory in different guns. If you are shooting longer range or with slower cartridges the difference is going to be more extreme. Consulting ballistic tables isn't enough, you have to get out and shoot your load in your gun to know what the trajectory is. If you just consult a chart you could easily be off by 6" or more if the range extends to 300 yards or farther.
Another example, over the weekend I was shooting with a friend and a new shooter, which is always great. He had recently tried handloading and had several different 223 cartridges loaded. I snagged some to see how they would group out of my rifle. He was happy to see they grouped better than cheap factory ammo. But the thing to note was the point of impact was about 2" off of the ammo I brought. That's a noticeable difference. The reason was bullet weight, I was shooting 69gr bullets and the ammo he made had 50gr bullets. It's always important verify where a given load shoots, because it may not shoot to the same point of aim as what you shot the last time. Plus, the more you shoot the better you will get. There is no replacement for experience and practice, so get out there and shoot!