This one’s personal. I’m Lance Rankin, owner and gunsmith at Western Sport in Roanoke, Texas, and Rankin Industries is my manufacturing brand; as a Type 07 FFL, I don’t just sell parts, I make them. The SP4 coated bolt carrier group is the product I’m asked about most, and it’s the one I’m proudest to put my name on. Here’s what it is and why I built it.
What SP4 is
SP4 is a hybrid DLC coating that came out of years of development. The problem with most thin film coatings, the common PVD and DLC finishes you see on boutique BCGs, is that you trade hardness for brittleness. Get it hard enough to resist wear and it can flake or crack under impact. SP4 solves that. It measures around 3000 on the Vickers hardness scale while staying flexible enough to take the beating a carrier absorbs on every cycle. Hard and tough at the same time, which is the combination that’s difficult to get.
The coating is a thin film between 2 and 3 microns thick with a friction coefficient of .02, which is remarkably slick. The same coating technology runs in race car engines and transmissions and has a solid reputation in NHRA and NASCAR circles. If it holds up inside a drag motor, an AR-15 is a comparatively gentle life.
Why you’ll notice it
Two things, mostly. First, cleaning. Carbon has a very hard time sticking to SP4. When it’s time to clean this BCG, you wipe it down with a dry cloth and the fouling comes right off. No scraping the tail of the carrier, no soaking, no wire brushes. If you’ve ever spent an evening fighting baked-on carbon, you know what that’s worth.
Second, lubrication. Through all the testing done on this coating, it runs best with no lubrication at all. It’s been run dry at 44 below zero in the mountains of Alaska and in the heat of Iraq and Afghanistan. It has also passed the ASTM 85 (Annex 1) salt fog test, so humidity and salt exposure don’t scare it. Dry parts don’t attract dust and grit the way wet ones do, and in Texas summer that matters more than people think. The low friction also means lower operating temperatures, smoother recoil, and less wear over the life of the group.
The build sheet
A great coating on a mediocre carrier is lipstick. So the underlying BCG is built to full mil-spec, HPT (high pressure tested) and MPI (magnetic particle inspected):
- Carrier machined from 8620 aircraft quality steel, heat treated to 60-61 RC
- Mil-spec chrome lined carrier interior
- Chrome lined gas key from 4130 steel, staked with Grade 8 screws torqued to 58 inch-pounds per mil-spec
- Bolt CNC machined from Carpenter No. 158 steel, shot peened for strength, with spring and ejector
- Tool steel extractor with spring and Viton O-ring
- Mil-spec firing pin (shrouded) and cam pin
- Full auto M16 profile carrier, fully compatible with semi-auto AR-15 rifles and pistols
- Weight: 11.40 oz
SP4 goes on every wear surface that matters: bolt carrier, carrier key, cam pin, firing pin, bolt, extractor, and gas rings. Not just the carrier body for looks.
Where it fits in the coating conversation
I’ve written a lot about BCG coatings because it’s one of the highest-value upgrades on the platform. If you want the full comparison of phosphate, nickel boron, nitride, and the DLC family, read my bolt carrier group coatings guide and my post on why BCG coatings matter. The short version: the BCG is the hardest working part in your rifle, and it’s where coating technology pays off most. That’s why I put my development effort here instead of on another muzzle brake.
Every Rankin Industries BCG is made in the USA and backed by a lifetime guarantee. If it ever fails to function properly, I’ll get you taken care of. That’s easy to promise when you’re the manufacturer and the warranty department is the same guy who built it.
It makes a strong heart for a new build too; see my AR-15 build guide if you’re starting from scratch. The SP4 BCG is available here on the site. Questions? Email [email protected] or call 817-393-4000 and you’ll get the maker, not a call center.
Lance Rankin is the owner and Chief Gunsmith of Western Sport, a family-run Type 07 FFL and Class 3/SOT dealer in Roanoke, Texas, serving shooters since 2007. Born and raised in Texas with over 50 years of experience handling firearms, Lance purchased Western Sport in 2017 and turned his lifelong passion into his profession. He specializes in AR-15 and AR-10 platform rifles, manufactures Western Sport’s own AR-15 line under the Rankin Industries brand, and runs the shop’s in-house Cerakote coating operation. When he’s not behind the counter or the workbench, you’ll find him on his ranch deep into the night, hunting coyotes with an AR-15 and thermal optics. Lance holds an MBA and personally shoots and tests the products Western Sport sells.
