lower receiver
├── magwell
├── grip
│ └── grip wrap
└── 🔑 fire control group
└── 🔑 buffer tube
├── stock
└── 🔑 buffer spring
└── 🔑 buffer
└── 🔑 upper receiver
├── 🔑 bolt carrier group
│ ├── 🔑 barrel
│ │ ├── 🔒 gas block
│ │ ├── handguard
│ │ │ ├── iron sights
│ │ │ ├── rails
│ │ │ └── underbarrel
│ │ └── muzzle device
│ └── charging handle
├── sights
└── brass catcher
🔑 — key component, the firearm won't work without it
🔒 — final component needed to operate the firearm
In Gunsmith, as in the U.S. law, the lower receiver is the firearm. Lower receiver is a structural piece. However, it's critical to the assembly of a working firearm.
Fire control group (FCG) is an assembly of trigger and hammer. The pull of the trigger is what leads to the hammer striking upon the cartridge inside the chamber, igniting the primer which ignites the gunpowder, the explosive expansion of which pushes the bullet out of the barrel.
Buffer tube is what holds a buffer. It's a structural component that stocks attach to.
Buffer spring maintains the operation of the buffer by springing it back once it's been pushed in.
Buffer is a heavy tube containing separate metal weights. The overall weight of the buffer defines how much response there is to the physical impulse of the bolt carrier group striking back upon firing the cartridge. In other words, it's what defines the gun's response to the immediate recoil of the shot.
Upper receiver is a structural component that hosts most of the parts directly interacting with the cartridge and the fired bullet. It commonly comes with a set of Picatinny rails useful for attaching sights.
Bolt carrier group (BCG) is an assembly of the bolt carrier and the firing pin. Together, they're responsible for managing the recoil from the fired cartridge in a controlled manner, as well as cycling the empty case out and a new cartridge in.
Upon firing, the recoiling BCG presses against the buffer, which presses against the spring, which pushes both of the parts back into their working positions. When fully open, BCG ejects the case through the ejection port. Pushed forward, it catches the next cartridge in the magazine and pushes it into the chamber, where the cartridge sits ready.
BCG is in large part responsible for the caliber the rifle can fire. Changing the BCG is the first step to changing the caliber of the rifle.
Barrel is the tunnel the bullet uses to escape the rifle from the pressure of the quickly-burning gunpowder. As the name of the firearm implies, its barrel is rifled – that is, has grooves cut on the inside. These grooves help twist the fired bullet during its travel, which helps it maintain its trajectory and not get swayed by external forces, such as the wind, on its path to the target.
The length of the barrel has a significant effect on the ballistics of the bullet. Longer barrels give more time for the bullet to accelerate, which increases its range, and improves both its accuracy and its terminal ballistics: that is, its strike on the target.
Barrel is another crucial component for changing the gun's caliber: bullets of a sufficiently-different caliber would either get stuck in or damage the barrel upon firing. Picking the right barrel is necessary to fire a different caliber.
Gas block is installed onto the barrel. It takes some of the gas propelling the bullet through the barrel and redirects it back into the mechanism. These gases are what pushes the BCG back and helps initiate cycling.
Installing the gas block is the last part required to operate an AR-15 in Gunsmith. Parts improving handling – such as the grip and the stock – are not necessary, but are highly recommended for installation.
Grip, referred to as pistol grip as opposed to a stock grip as seen on many sniper rifles, is the part you hold onto with one hand while handling the rifle. Grip provides easy access to the trigger.
Grip wrap may be used to enhance handling of the grip. It often provides additional texture, which makes it slightly easier to manage recoil.
Magwell parts are often magazine adapters, which are installed when the caliber you want the rifle to fire uses a smaller magazine than the lower receiver is able to fit. Installing a magazine adapter for allows the gun to feed ammunition from a new type of magazines.
Stock presses against the user's shoulder, softening the impact of recoil and helping maintain aim after firing.
Sights help user aim more precisely. They're used as a replacement for or in conjunction with iron sights. Some models even use the iron sights as a base reticle while highlighting it or augmenting one's aim in other fashion.
Telescopic sights, often referred to as scopes, provide magnification, allowing user to see targets clearer from further away. Different kinds of non-telescope sights exist, primary in the form of reflex sights aka red dot sights, which display a tiny bright dot on a lens inside the sight.
Charging handle allows manually cocking and cycling the rifle. It's primarily used when inserting a new magazine, when there's no returning gas to push the BCG and force a cycle yet.
Brass catcher is fitted against the ejection port in order to catch ejected cases. This is useful on a firing range, where post-shooting clean-up could be made that much shorter, or in the wild if you're looking to safe the cases for reloading.
Handguard is the part covering the barrel. User would hold onto the handguard while handling the rifle, which enables better control of the gun. It also prevents user from burning their hand on the barrel, which gets very hot after firing due to the expanding gases covering the barrel's length. Handguards often provide rails or rail mount locations.
Iron sights are the basic sights of most guns. There could be simple or somewhat sophisticated. Iron sights are installed in pairs: one in the front and one in the back. In an AR-15, the front is the farthest edge on the upper side of the handguard, and the back is the nearest edge of the upper receiver.
Rails provide surfaces to which various attachments can latch onto. These include laser sights, flashlights, and other types of useful tools. Many modern handguards have four separate rails: left, top, right, and the bottom aka underbarrel.
Underbarrel is often a rail dedicated to specific types of attachments. These include underbarrel grenade launchers, underbarrel shotguns, forward grips, and other types. Short underbarrel attachments, such as most forward grips, leave enough space for another rail attachment.