If you're new to the firearms community and looking to build your first AR, it can feel overwhelming at first. AssembleAR is designed to make finding parts and ensuring you have everything you need as simple as possible.
At its core, an AR is composed of an upper and lower receiver held together by takedown pins. The lower receiver is the only serialized part — the one that must be shipped to a licensed dealer and requires a background check. Every other part can be shipped directly to you.
Before You Start
Legal Disclaimer
⚠ Important
The AR platform can be configured as either a Rifle or a Pistol. You are solely responsible for complying with all federal, state, and local laws. Some states prohibit the AR platform entirely or restrict pistol configurations. Always check your local laws before purchasing.
AssembleAR provides this guide for educational purposes only. We are not legal advisors. When in doubt, consult a licensed firearms dealer or attorney in your state.
Parts Guide
Overlapping Parts
Several items in our parts list include multiple components. Ordering these can eliminate the need for individual parts — but you need to know what's included to avoid buying duplicates.
Complete Upper Receiver
A complete upper already includes the barrel, handguard, and often the BCG and charging handle. Read the listing carefully — if it says 'w/o BCG / w/o Charging Handle' you still need to add those separately. If the BCG and charging handle are included, remove them from your list.
Complete Lower Receiver
A pre-assembled lower already includes the trigger group, lower parts kit, and sometimes a stock. You won't need individual parts from the trigger down to the grip. You may still want to upgrade specific parts like the trigger, but it is not required for the lower to function.
Receiver Set
Ordering a receiver set gives you both the upper and lower receiver together. You will not need to add a separate upper receiver or lower receiver to your parts list.
Buffer Tube Kit
Includes the buffer tube, buffer weight, and buffer spring — the three parts that make up the buffer assembly. If you order this kit, remove each of those individual items from your list.
Lower Parts Kit (LPK)
Includes the springs, detents, and takedown pins needed to assemble a stripped lower. Most LPKs include a mil-spec trigger, bolt catch, magazine release, and end plate — but not all. Read the description carefully. Parts like the trigger, bolt catch, and end plate are listed individually because many builders prefer to upgrade them.
✓ Tip
When in doubt — read the product description twice. Every overlapping part is only a problem if you accidentally order duplicates.
Caliber Guide
Choosing Your Caliber
The AR platform supports a wide range of calibers. Understanding the differences will help you pick the right barrel and ensure every part in your build is compatible.
5.56 / .223 Wylde
The standard AR caliber. A 5.56 barrel accepts both 5.56 and .223. A .223 barrel accepts .223 only. .223 Wylde accepts both and is optimized for precision shooting.
.223 Rem
A .223 barrel can only be used with .223 ammunition — not 5.56. This is due to the higher chamber pressure of 5.56 NATO. Mark your barrel clearly.
300 AAC Blackout
Requires a 300 BLK barrel, but every other AR part remains the same — including the BCG. Some BCGs are not rated for 300 BLK, so read the description.
.308 / AR-10
.308 ARs require a different upper receiver, lower receiver, BCG, buffer weight, and buffer spring. Do not mix AR-15 and AR-10 parts.
⚠ Important
Never fire 5.56 ammunition through a .223-only barrel. The higher pressure can damage the barrel or cause dangerous failures.
Gas Systems
Understanding Gas Lengths
The AR uses a direct impingement gas system to cycle the action. Gas length determines where the gas port sits on the barrel — which affects reliability, felt recoil, and wear. Your barrel, gas block, and gas tube must all match.
Pistol Length
Under 10"
Very short dwell time. Used on pistol-configuration builds with braces.
Carbine Length
10" – 18"
The most common gas length. Works well on 14.5" and 16" barrels.
Mid Length
14" – 18"
Softer recoil than carbine. Popular on 16" barrels for a smoother shooting experience.
Rifle Length
18" – 24"
Designed for full-length 20" barrels. Smoothest recoil of all gas lengths.
ℹ Info
Gas block diameter matters too. The standard diameter is 0.750". Some barrels have a stepped or thicker profile that requires a different diameter. Always check the gas block diameter against your barrel spec before ordering.
A mismatched gas system is one of the most common causes of reliability issues in AR builds. If your rifle fails to cycle or ejects brass erratically, gas length is often the first thing to check.
