It looks like their MEDCON Barrel
I have their 16" and it's a pretty good piece of hardware
Rainier Arms Select™ 5.56MM MEDCON Barrel - 18"
There are a number of advantages to polygonal rifled barrels; such as less bullet deformation, resulting in reduced drag as well as prolonged barrel life, not to mention ease of cleaning. These barrels were made to our exact specifications & drawings and made of 416R Stainless steel. We control the manufacturing process to provide the most accurate barrel in its price range. We are so confident in our Select grade barrels that we will offer a 100% money back guarantee within 90 days of purchase, no questions asked. If you aren’t completely satisfied with the barrel for any reason, return it for a full refund. Guaranteed to shoot sub-MOA at 100 yards with match ammo. You deserve this confidence inspiring barrel for your next custom build.
We took as much weight off the barrel as we could without sacrificing accuracy. We don't save a lot off the standard barrel (little over 1 oz). But for guys who are building the most accurate rifle while keeping weight down, this is the perfect barrel for you.
416 R ordnance grade stainless steel produced in Germany by the same mill as H&K
F bar rated = meaning it is acceptable for use by all Nato country’s
Extensions made from 4140 AE = meaning aircraft grade steel
Manufacturing proprietary processes creates a stress free product that has a hardened surface on the bore. To improve the wear and longevity
M4 feed ramps on extensions to current production standards to ensure conformance between manufactures
Barrel Length: 18
Gas Block Seat: .750
Rifling: Polygonal 5 Groove
Weight: 33.3 oz
Stainless Steel: 416R
Feed Ramps: M4
Gas System: Midlength
Finish: Matte Stainless / Black Oxide
Thread Pitch: 1/2×28
Testing: MPI tested
Made in the USA!