Readily available parts of all sorts make the AR pattern Rifle a Do-It-Yourself-ers Dream.
It is not at all difficult to assemble- any idiot can do it- and the intra-webz is full of examples of this fact.
things to make sure that you order for a full build
Lower Receiver: This is the only 'controlled' part- by Law, the Lower Receiver *is* the Rifle, and so, it must be purchased through an FFL, with a NCIC. The Quality of most available Lowers is outstanding- while there are always a few bad ones that slip through, the majority of 'Off the Shelf' (OTS) units are are of excellent quality. The primary variance seen between various suppliers seems to be the finish .
Lower Parts Kit: These are ALMOST as good as the Lower, quality control wise. Shop price, But look for a more established Supplier. The LPK includes a trigger guard, Fire Control Parts (Trigger/Hammer/Selector + springs and Pins) Pivot/take-down pins (and springs) Mag Catch, and Bolt Catch (both with pins and springs), Buffer retaining pin and spring, and usually a Pistol grip and Pistol grip Mounting screw.
To complete a lower assembly, you will need a Receiver Extension (aka Buffer Tube) Buffer, and Buffer Spring, and a Buttstock. Telescoping M4 type Buttstocks require a Castle Nut (usually included). Receiver Extensions come in either 'Mil-Spec' or 'Commercial' sizes, there being a slight Difference in O.D.. Springs and Buffers will fit either, But if you are not buying a complete assembly, do make sure that the stock and the Receiver Extension match.
To assemble these parts into a complete lower, you will need a pair of Water Pump Pliers (aka 'Channel Locks'), Masking tape, a tiny 'eyeglass' type screw driver, and a medium sized or larger plastic bag. A set of allen keys will serve as alignment tools.
A mounting block (The ones I've seen are basically Dummy Magazines that can be clamped in a vise) really seems unnecessary to me- ditto for hammers and punches. A Special Pivot pin assembly tool is nice, but I have always done fine with a tiny screw-driver. Instead of trying to drive pins in with a hammer, I find using a pair of WP Pliers as a hand press to work just fine, and it won't break or mar anything ( cover the jaws with Masking Tape). For a M4 type stock, you WILL need a hammer and Punch to 'Stake' The Castle nut
Most of the assembly should be done inside of the plastic Bag- especially when dealing with the teeny-tiny Pivot pin detents and springs.
Upper Receivers come in 3 basic varieties- M16 (no fwd assist or brass deflector, with Carry Handle) A2 (FA & Brass Deflector with a Carry Handle) and A4 (FA & Deflector, with 'Flat top' Rail). Some (most?) will have 'M4 type' feed ramps, which should be matched with a barrel that has them as well. Standard Feed ramp Upper with a M4 Feed ramp Barrel is OK, But M4 feed ramp Upper with standard ramp Barrel is not OK.
A complete Upper Receiver has a Bolt Carrier Group, a Charging Handle, A Dust Cover (with pin, Spring, and C clip) and a Forward Assist (with Spring and Pin). A 'Stripped' Upper is just the Upper Forging.
For a Complete Upper Assembly, you will Need a Barrel (almost always sold assembled with a 'Barrel Extension'- the part that the bolt engages), a Gas tube, a Gasblock (or Gas Block/Front Sight Assembly aka FSA) a Gas Tube Pin, a Barrel Nut, and a handguard Assembly- for the Standard type, you will need a Delta Ring, Spring, and snap ring, and a Handguard Cap. A 'Free Floating' Handguard will often require (and is usually sold with) a specialized Barrel Nut, or some other specific hardware.
You may need a Muzzle Brake, depending on your Barrel Choice. Gas Blocks or FSAs require either Pins or Set Screws. Handguards and Gas tubes should be matched to Barrel Gas System Length (Carbine, Middle, or Rifle). Assembling the barrel to the upper (and an M4 type Buttstock) is easier with a 'Armorers Tool' but you can usually get it done without one.
If you don't have 'Calibrated Hands', you'll want a torque wrench, or a friend with calibrated hands.
I always take all the parts, Slather them with CLP, and let them sit like that for a day or so, sometimes even heating and cooling them (MILDLY! like front the freezer to direct sunlight for a few hours) to let the CLP fill the pores, then wipe them dry before assembly. This is sometimes called 'oil buffing'.
Chrome BCGs are wonderful things, IMX. WAY easier to clean and clean well than standard coatings.
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