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.
18" SPR Ballistic Advantage 5.56 Barrel, 15" Breek Arms Mlock Rail, Toolcraft NIB BCG, Mil-Spec Upper and Charging handle
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)
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.
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.
Material - Steel
Style - Low Profile
Size - .750"
Attachment Method - 2x Set Screws
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.
Length - Rifle
Material - Stainless Steel
A2 Flash Hider - very basic flash hider
MK4 - 18" 5.56 SPR Precision Upper Receiver - Pinned Gas Block
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.