First of all, you need to understand that tuning will very much depend on the goals. Beginners usually consider tuning as an increase in their combat capability. That is, as an alternative to training with firearms. Well, like I bought a tactical grip and got +30 to speed and accuracy. That is, if a dangerous situation suddenly arises, then the carbine will practically jump into your hands and aim at the enemy. No, it won't hover and it won't jump. And no body kit will help this if you do not periodically shoot.
Some people initially and are not going to shoot from a carbine. Well, they don't plan to do it often. They like the process of tuning more. They "dress up the Christmas tree." Well, why not? But, if you are still going to shoot at least sometimes, but don’t know what to choose, get an inexpensive version of the AR-15. Structurally, they are all the same, and if necessary, you can easily improve your inexpensive device to the level you need.
First of all, consider purchasing basic things, such as sights. A lot of carbines come without them or only with mechanical ones and a front sight. If you have enough of them, fine. But we would still advise you to purchase an inexpensive red dot sight. Make sure there are magazines in the kit. Might have to buy them. Often, a 10-round magazine is included in the kit, and you want a larger capacity magazine. Take it! For transportation, you will need a case - the firearm must be unloaded and cased when you transport or carry it. And from the point of view of the law and from the point of view of common sense.
Think cleaning supplies. Are you planning on shooting? Cleaning rod, vishers, patches, synthetic neutral oil, copper remover. Do not take a lot of things at once - a part will then lie idle. Better then buy more when you realize that you are missing something. It is very important to use your eye and ear protection when shooting. Choose comfortable goggles that fit comfortably on your face. They won't fit very tightly, as they will fog up and have temples thin enough not to put pressure on the head. Especially when they press on top of the headphones. Headphones are more comfortable when they are active - this is a fact, but normal ones are also suitable for a start, look at the budget.
So, let's say you've got everything you need to start owning your AR-15, how can you improve it now? Let's start with ergonomics. There is no single recipe in this matter, since we are all very different anatomically. To better understand what you are missing, analyze your stance and tab and see what moments in them you can make more comfortable for yourself by changing one or another part of the carabiner. Pay attention to the length and shape of the forearm. Is it comfortable? Perhaps you need a buttstock with a cheekpiece or a different height of the riser of the sight if you have to “steering” your head with each throwing up in order to see well into the scope. Or a thicker grip if you have long fingers and your index finger gets too deep on the trigger. Maybe you do not like the backlash of the upper and lower, which can be removed with special pins or a banal rubber spacer between them. For a faster and more confident sending of a cartridge into the chamber, it most likely makes sense to change the cocking handle so that the new one has a more grippy lever. Or just change the lever on the native handle, as it is much cheaper, and the work takes five minutes. An oversized magazine reset button is not only useful, but also harmful, as it increases the risk of accidentally resetting the magazine. The ambidextrous safety lever can be unexpectedly uncomfortable due to the fact that it will press on the base of the index finger of the right hand of a right-hander.
The next thing you should pay attention to is the reduction of recoil and muzzle flip when firing, as well as the comfort of trigger handling. If your AR doesn't have a compensator muzzle brake, then you should start with it. It significantly reduces the recoil of the carbine and the rise of the barrel by redirecting the reactive jets of powder gases emitted from the barrel. Perhaps the most popular is the SJC Titan and its many replicas. Not only because of its performance, but also because of its appearance and digestible dimensions. Also popular with JP Eliminator athletes. But these are not the only options, there are a huge number of them on the market. We will talk about some of the nuances that you should know about. Any DTK, even a mediocre one, will greatly improve your shooting experience. Keep in mind that when installing any muzzle device, you will have to zero in the carbine again. Many shooters get used to their DTC and subconsciously begin to compensate for the micro-movements of their DTC. Therefore, when changing it, even for the best model, discomfort and deterioration of results may immediately appear, since it takes time to get used to the new product. Also, many put DTK at an angle. Firstly, when shooting, many people feel an even greater effect of the DTK when the upper nozzles are not directed vertically upwards, but at an angle to the right. And secondly, if there is an additional side collimator sight on the carbine, then such an arrangement of the DTK works more for it. you need to remember that when shooting prone with the DTC tilted to the side, part of the gases on one side will be directed to the surface on which you are lying. Accordingly, dust and dirt can fly to the sides and on you, lifting a curtain that will interfere with your shooting and smear your sights and glasses.
Next is the trigger mechanism (USM). Regular triggers on inexpensive models are usually rigid, with a long stroke and a large trigger pull. For beginners, this is justified - when learning, there is less risk of making an accidental shot. But for a trained shooter, priorities change. He already has a solid trigger control skill, and wants to achieve a smooth trigger for more accurate shooting and a short reset for faster splits. There is a common recommendation that the escapement should be soft and short and resemble a “breaking glass rod” in its work. Special sports triggers for the AR-15 are not cheap. An alternative to an expensive USM is weakened springs for your stock. They are inexpensive and greatly improve the feeling when shooting, but the spring can sag and because of this, situations occasionally occur when a shot does not occur due to the fact that such a spring is not able to hit the primer with sufficient force.
As a rule, the correct DTC and USM will dramatically change your results and feelings from dynamic shooting. Many stop there. But in the matter of tuning there is much to go further. The decrease in firearm vibrations during firing can be reduced by reducing the masses and velocities of the carbine parts involved in reloading the firearm. Simply put, after each shot, the bolt group and recoil buffer fly back into the butt tube, extracting the empty cartridge case from the chamber, and return back, sending a new cartridge from the magazine into the barrel and locking the bolt. These rather weighty "pieces of iron", flying back and forth, significantly affect the "twitching" of the firearm in your hands. Which, in turn, causes fluctuations in the picture that you observe through the sights. But you need to understand that by interfering with the operation of the carbine's automation, we are on a slippery slope choosing between the comfort of shooting and the reliability of the firearm.
So, the next step is to replace the standard gas block with an adjustable one. What is in this question? The fact that the factory settings of the carbine is the so-called "overgas" when reloading. That is, the stock gas block removes significantly more gases to reload the carbine than is necessary. The manufacturer does not know what cartridge and under what conditions you will shoot, so it is reinsured and "blowing" in full. So that even a dirty carbine in combination with a low-quality cartridge can be reloaded. If you're willing to take the risk, then with the help of an adjustable gas block, we reduce the gas supply to reduce the force with which the moving parts accelerate after the shot and hit the rear end of the butt tube. You will notice changes in the operation of the carbine not only by your personal feelings, but also by the angle of departure of the spent cartridge case, it will fly out more to the side.
The main idea of all these activities is to make sure that your carbine practically does not move when shooting and the effort required for the descent introduces a minimum of fluctuations into your carbine-shooters system.
You can also get carried away with lightening your rifle. This is the kind of tuning where aerospace prices buy the lightest carbine parts to save a few extra grams of weight. The idea itself is interesting, but requires a decent budget. In addition, it may rather be of interest to the shooter, who wears a carbine for a long time. But it is worth remembering that you can’t deceive physics - the lighter the carbine, the more mobile it is when shooting, and for comfortable shooting it is often not the weight of the firearm that is more important, but its balance.
And of course, the appearance, the AR-15 platform allows you to make a rifle of any color, give it an individual style. It's hard to give any advice here - everyone has their own vision of their ideal firearm, but in any case, when doing something with your AR-15, understand why you need it.