Upper receiver now days can get extremely fancy, so for budget options, I recommend the standard BCM if you can find one available. Aero Precision is also a good option, but the tolerance can be off sometimes compared to something like a Daniel Defense. Anyway, here are some good budget options:

• Aero Precision M4E1
• Anderson
• BCM

For higher end options:

• VLTOR MUR upper
• Radian Upper
• PSA JAKL

AR15 Bolt Carrier Group Options

AR15 bolt carrier group is one of the major parts that makes the rifle run. Most AR15 BCGs on the market today are full auto rated, but this doesn't mean the rifle is full auto without the full auto trigger system

There are many options available with various coating (Smooth cycling definitely matters) and for a budget build, these are recommended:

• BCM
• ToolCraft
• PSA
• Faxon gunner ultra light weight BCG
• Brownells Chrome BCG
• FailZero EXO BCG

On the higher end. These are recommended:

• Sons Of Liberty Gunworks BCG
• 2A Armament lightweight BCG
• Sionics NP3 BCG

AR15 Barrel Options

For a legal barrel length AR15 chambered in 5.56, a 16" long barrel is required (Not accounting for the length of the muzzle device). There are many AR15 barrels available, and these are our recommendations between budget to mid price range.

For 16" barrels. Ballistic Advantage 1:8 Hanson profile barrel is the best without spending a ton of money. The barrel however requires dimple to secure the gas block. The 1:8 twist rate is best for 50, 55 and 62 gr bullets, and 62 grain bullet gives you some stopping power to knock down steel spinner targets for competition shooting without wasting ammo.

1:7 twist rate is good for 77 grain bullet if that's what you want to shoot.

For the higher end stuff, here are the best in the price range of $250 - $300

• Daniel Defense cold hammer forged barrel
• FN cold hammer forged barrel

The options are endless, and this guide can't include all of them, please see the AR15 barrel guide to see more

For 300 Blackout barrels please go here

Gas System

For a direct impingement AR15 build, there are so many options available for the 5.56 platform, and they all do the same thing. If the handguard goes over the gas block, be sure to use a low profile gas block and either pin it to the barrel, or tighten it with a set screw where you dimpled the barrel.

0.750" diameter size gas block is the most common for most barrel profiles. Just buy the gas tube that fit the gas length of your barrel. Aero precision offers the most affordable gas block and gas tube:

• Carbine (Pistol) length
• Mid Length
• Rifle Length

To learn more about AR15 gas tube length here

AR15 Charging Handle

The Radian raptor charging handle is the best. It offers the best ambi control feature among others, and its the most popular.

If you want something else, these are your options:

• BCM Gunfighter MK2
• Aero Precision Breach

They are all round the same price range and some versions are cheaper.

AR15 Handguard Options

There are plenty of AR15 handguards available. MLOK is the most popular today because it saves a lot of weight compared to M1913. Keymod is still popular.

As far as building the most rigid handguard system. The Geissele and BCM offers the best products. 

• Geissele MK16 handguard
• BCM 9" MCMR

Check out more AR15 handguards here

In the context of building a 16" barrel AR15, a 9 inch or 13 inch handguards are my personal favorite because they provide just enough rail space. Anything longer than that maybe wasteful if you don't plan on mounting accessories.

AR15 Builder Tools

AR15 upper parts are fairly simple to put together. While they are marketed as "LEGO" alike, but there are components that need careful tolerance check and alignment for the perfect build.

Here are must have tool to get:

• Midwest Industries Reaction Rod - Ensure barrel extension and index pin alignment before applying torque on barrel nut
• SAAMI Go/No Go gauge - Ensure the proper and safe chambering of rounds in firearms
• Vice clamp - A work bench is needed to mount the vice clamp to torque the barrel nut and muzzle device

When you're in the market for a new gas block for your firearm, it's crucial to match it to your barrel's diameter. Typically, you'll encounter gas block sizes denoted in inches, like the common 0.750". This measurement should correspond to the width of your barrel to ensure a proper fit.

Based on School of American Rifle's teaching, when it comes to gas efficiency and bolt travel speed. The tightness of the gas block around the barrel seal is also a factor to prevent leakage. 

The same goes for the gas pressure around the gas ring and bolt tail. So this means the buffer system and the bolt carrier group are also important parts.

Any leakage around these areas can affect the rifle's operation besides just picking the buffer weight and the buffer spring.

If you have a barrel that's on the slimmer side, reminiscent of the vintage M16A1, you'll be looking for a 0.625" gas block. On the other hand, for those beefier, thick-barreled rifles, a 0.936" gas block is your go-to.

Gas Block Considerations

Adjustable Gas Blocks

Adjustable Gas Blocks allow you to tweak the amount of gas that cycles through your firearm, especially when you're using various types of ammunition especially when shooting suppressed. This flexibility can be a real game-changer in preserving the lifespan of your gun's components. It's like having a customizable setting for different shooting scenarios.

Low Profile Gas Blocks

Thee low profile gas blocks are very popular for AR15s with free float handguards. Because they fit snugly under those rails without any contact with the rail, and it provides a seamless integration that doesn't mess with your your firearm. It's all about maintaining that perfect balance between functionality and form.

Lightweight Gas Blocks

These are a top pick for competitive shooters. In shooting competitions, every ounce counts. Opting for a lighter gas block means your rifle won't be weighed down, giving you that slight edge in speed and maneuverability.