MK4 - 18" 5.56 SPR Precision Upper Receiver - Pinned Gas Block

The MK4 is a precision oriented upper receiver. Accuracy and consistency is the main focus here. No frills, just the most precision you can squeeze out of 5.56 AR platform with out the price tag. I guarantee Sub MOA 5-shot groups with high quality ammo.  In my personal testing I have gotten right at 1 MOA 10-shot groups. These are gassed to perform well with or without a suppressor. If you plan on using a lot of steel cased ammo we recommend Carbine Buffers and Springs. If you are running suppressed or only plan on using hotter ammo you may want to increase your buffer weight, again, depending on how you plan to run them. These are not lightweight uppers as the barrels alone come in at just over 3lbs.


The Basics


18" SPR Ballistic Advantage 5.56 Barrel, 15" Breek Arms Mlock Rail, Toolcraft NIB BCG, Mil-Spec Upper and Charging handle


The Specifics



Manufacturer - Ballistic Advantage SPR

Length/Chamber - 18" 5.56

Twist - 1:7 Twist

Material - 4150 Chrome Moly-Vanadium

Profile - SPR with OPS 12 Compatible Collar

Gas System - Rifle 

Finish - Nitride

Thread Pitch - 1/2x28

Gas Port - Drilled to just above MK12 Spec .1065"

Dimpling - Yes

Weight - 41oz

1st Rebarrel - FREE


Barrel nut is sequentially torqued down to 50 FT-lbs. Starting at 25, then 35, and then finally 50 FT-lbs. to ensure proper mating of the threads. 


If you legit burn out a barrel, like 10,000 -25,000 rounds, and the barrel becomes inaccurate I will rebarrel it for you for free. Good luck.



Bolt Carrier Group:

Manufacturer - Toolcraft

Bolt Material - 9310 Alloy Steel 

Carrier Material - 8620 Alloy

Gas Key Material - 4130 Alloy

Profile - Full Auto

Finish - NIB

Gas Key Screws - Grade 8 (torqued and staked to Mil-Spec)

Forward Assist Cuts

Mil-Spec Firing Pin (Hard Chrome Coated)




Manufacturer - Breek Arms

Length - 15"

Material - 6061 T6 Aluminum

Finish - Hard Coat Black Anodization

Mounting Platform - 3-Sided Mlock

Barrel Nut - Steel (does not require timing)


Upper Receiver:

Manufacturer - Anderson

Material - 7075 T6 Aluminum

Machined from Forged Aluminum

Finish - Type 3 Hardcoat Anodized Black

Standard Forward Assist

Standard Dust Cover

M4 Feed Ramps

NOT T-Marked, if you care...


Nothing special here, everything is Mil-Spec (standard), which is the way that I like it for an upper receiver.


Charging Handle:

Material - 7075 T6

Machined from Forged Aluminum

Finish - Type 3 Hardcoat Anodized + Teflon Coated for Smoother Operation


Same here, standard and boring. High quality, high strength, enough said.


Gas Block:

Material - Steel

Style - Low Profile

Size - .750"

Attachment Method - 2x Set Screws


Dimpled Barrel

Not Adjustable


You shouldn't really notice a good gas block and this one is no different. High quality steel that's as tough as the barrel for longevity. Rear set screw fits into the dimple on the barrel, ensuring proper alignment and that the gas block won't shift on you.


Set screws are torqued to 30 IN-lbs. and have High-Temp threadlocker applied for extra security.


Gas Tube:

Length - Rifle

Material - Stainless Steel



Muzzle Device

A2 Flash Hider - very basic flash hider


MK4 - 18" 5.56 SPR Precision Upper Receiver - Pinned Gas Block

Out of Stock
  • Our stated policy is to not accept returns, but if you accidently ordered the wrong item or there was a mistake or if you have any issue at all send us an email (listed in the "contact us" section) and we will work it out.