My parting gift. One of my baes asked me to teach them how to build a firearm, so here is my semi-basic tutorial on how to create an M4-style assault rifle.

Sections have been color-labeled for convenience:
Dark Blue - Body
Light Blue - Magazine
Pink - Stock
Purple - Handguard
Red - Barrel
Gold - Accessories

The main body of the gun is by far the most complex part. It contains the upper and lower receivers (note the slight color difference differentiating the two). The upper receiver has the priming mechanism (charging handle, bolt, firing pin, etc.) and the shell ejection port, when spent casings are ejected out the sides.
The lower receiver has a lots of parts. Some details you should include are the trigger, safety, fire mode selector, and mag release button.
When you’re building the basic shape of an M4 body, the biggest thing to note is the circular section. My advice is, try starting with two 2x2 round fuselage sections and build off of that.

The magazine feeds rounds into the receiver, typically 30. There are plenty of different styles and shapes, including short 20 round straight mags or high-capacity drum mags. For your standard M4 magazine, it’s very easy to build: Just start with a long, straight fuselage block and add downward, putting the blocks at an increasing angle.

The stock helps provide stability when firing. The long round piece attached to the back of the receiver is called the receiver extension, which the buttstock would slide onto IRL. I actually have a really hard time building weapon stocks, for some reason, so it’s best to try and replicate a real model. I suggest Googling “m4 stock” to get a good variety of fixed, telescopic, and skeletal stocks.

The handguard probably has the widest diversity. Its actual purpose is to prevent your hand from touching the barrel, which gets really hot after shooting through a mag or two. While an M16-style handguard is smooth, a tactical handguard usually has four rails to attach additional equipment. The bottom rail takes a foregrip, bipod, or grenade launcher, laser usually gets placed on the top rail, and it’s not irregular to leave the side rails clear or cover them with rail covers.
Basic handguard is large round fuselage blocks with long flat pieces set on the outside. Take a look at my HK416-C for my most detailed rails.

The barrel is important in defining the role of the firearm. In a real gun, it would stretch through the handguard, all the way to the upper receiver.
A shorter barrel denotes a carbine. A longer barrel denotes a DMR or a SASS.
If you’re going to build a large-calibre weapon (battle rifle or sniper rifle with the same basic setup) the barrel diameter should be larger as well.
Needless to say, this is the easiest part of the gun to build. A line of 1x1 round fuselage blocks should do the trick.

There are like a billion different accessories you can brighten up your gun with. On your top rail, you want either iron sights or an optic. Common RDS brands include Trijicon, Aimpoint, and EOTech. Laser sights look good for military rifles, a good example being the AN/PEQ series of visible or infrared lasers. Flashlights also have their place, try the Streamlight brand for inspiration Vertical or angled foregrip help with stability, and the aptly-named “grippod” looks fantastic on a tactical build.
There’s also a lot of room for variation in the front of the barrel. A standard M4 comes with a basic flash suppressor, but certain builds look great with muzzle breaks or suppressors.

The best way to learn is to take my model apart and then try building it yourself!