MK5 - 12.5" 5.56 Complete Upper Receiver - Pinned Gas Block

The MK5 is our last upper for the MK series of uppers, with more improvements to the overall build. These are Do-It-All uppers build to be extremely accurate, reliable, and affordable. First rebarrel is still free and every component has a lifetime warranty.


Each upper ships with a unique checklist noting the ejection pattern with brass  ammunition, function, lock back on empty, and any buffer/spring recomendations I may have.


The Basics


12.5" Ballistic Advantage 5.56 Upper, 11.7" Breek Arms Mlock RAill, Toolcraft NIB BCG, Mil-Spec Upper and Charging handle


The Specifics



Manufacturer - Ballistic Advantage

Length/Chamber - 12.5" 5.56

Twist - 1:7 Twist

Material - 4150 Chrome Moly-Vanadium

Profile - Government

Gas System - Carbine

Finish - Nitride

Thread Pitch - 1/2x28

Gas Port - .074


*Pinned Gas Blocks*


1st Rebarrel - FREE


All NFA/ATF Rules Apply


Ballistic Advantage makes good stuff and these are no different. The 12.5" Performance series is gassed well for all ammo types with a .074" gas port on the Carbine gas system. Recoil is negligible and ejection and extraction is strong and consistent. I recommend a standard carbine spring and buffer for steel cased or an H1 or H2 for the hottest of ammo, though you can always tune to your desired feel. These are very accurate barrels, I expect around 2MOA accuracy with good ball ammo and 1 MOA or better with good match loads.


Barrel nut is sequentialy 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 - 11.7"

Material - 6061 T6 Aluminum

Finish - Hard Coat Black Anodization

Mounting Platform - Mlock

Wedge Lock Attachment


Barrel nut torqued to 50 FT-lbs

Wedge Lock Tightened down to 60 IN-lbs with Blue Loctite


The Build Process:


Every upper is assembled by me, one at a time. Every piece is hand fit, barrels to the uppers, gas blocks to the barrels, gas tubes to the blocks, handguards, roll pins. If something doesn't fit properly and securely, I don't put it together. What that means is that you get an upper where every component fits together like a glove for a rock solid build that won't fall apart or tear itself asunder during hard use.


On top of that we use the proper threadlocker on the appropriate parts. We use blue loctite on the handguard screws and then torque them to 35 IN-LBS. On the gas block screws we torque them to 25 IN-LBS then use a red high temp threadlocker (#272) to ensure the gas block never shifts. #272 threadlocker holds its strength up to 450 degrees VS. the 300 degrees of standard loctite. 





MK5 - 12.5" 5.56 Complete 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.