My quest was simple. To build a suppressed AR15 that weighs under 8 lbs unloaded. Since I was using a JP Low Mass Bolt Carrier, I knew I'd need some way to keep the gun from overgassing the hell out of the bolt carrier with the Surefire 762Mini attached. I looked at various adjustable gas blocks, but I didn't like their use of tiny coil springs and detents in one of the hottest areas of the gun. Heat and springs don't mix. I wouldn't be adjusting it often as the suppressor would almost never leave the rifle, but I wanted something I wouldn't have too constantly worry about.
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| SLR Sentry6 |
Enter the SLR Sentry6. Unlike other gas blocks, this utilizes a flat leaf spring tempered to 1400 degrees F. Being external, it is exposed to cool air and is easy to rebuild if needed. Everything is finished in Melonite QPQ to avoid corrosion, and the adjustment screw is in the front of the block, making adjustment a snap under any free-float rail with a long enough hex driver. The black finish is smooth and even, and there is a starter hole on the right side for drilling and pinning. There are also three vertical lightening cuts in the side, though probably more for aesthetics than weight savings. SLR's Dimpling Jig made dimpling my nitrided barrel a snap with a hand drill. I recommend a cobalt drill bit to make short work of the hard finish. The Sentry6 slid easily onto the barrel with a little wiggling and fit snugly with zero perceptible wobble. I added a dab of Rocksett to the set screw threads with a toothpick and secured them at 25 in/lbs. Next I setup the barrel and gas block under a drill press and drilled the .125" retaining pin hole. This is considered by many to be unnecessary. While taper pins are the strongest way to secure a gas block to a barrel, they are used with a traditional FSB, where the front sight tower will likely get knocked around. For a gas block protected under a rail, set screws and Rocksett should be secure enough. However, I like my stuff to be bullet-proof. I opted for stainless steel solid groove pins. These are much stronger than a roll/spring pin so heat shouldn't be an issue at all. The pin is actually slightly tapered, but unlike a traditional taper pin, three grooves are cut into the sides allowing it to compress inside a straight hole, so you can use a standard drill bit instead of a taper reamer.
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| SLR Sentry6 - Gas Screw |
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| SLR Sentry6 - Grooved Pin |
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| SLR Sentry6 - Bottom |
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| SLR Sentry6 - Leaf Spring |
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SLR Sentry6, MI 10.5" G3 Rail, Faxon 11.5 Mid-Length, Surefire SFMB556
This setup gives easy access to the adjustment screw |
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SS Grooved Pins - McMaster-Carr Part# 98400A602
1/8" Cobalt Drill Bit - McMaster-Carr Part# 3069A16 |
The Sentry6 has 15 gas positions and makes 1 full revolution every 3 clicks. My setup is an 11.5" Mid-Length Barrel, JP Low Mass Carrier, 3oz CAR Buffer, and Surefire SOCOM 556 Muzzle Brake with Warden attached. I started at position 4 from closed. I had to open it up to position 7 until it would lock back on a magazine. With the Surefire 762 Mini attached, I had to turn the screw down to setting 4. I then added an extra click open for reliability in case the screw got fouled. More likely though, is that the metering screw will get eroded by hot gas and I'll actually need to close it some as it starts to over gas the BCG.
That was a few months ago. Since then, I replaced the 3oz CAR buffer with an H3. Shooting suppressed gets the action really dirty, and the extra weight of the H3 buffer helps with reliability after a few hundred rounds if I don't have a chance to cleam/lube the bolt carrier. I had to turn the gas screw out to 9 from closed. I'm also waiting on a Form 4 for a Griffin Recce5 Mod 3. It, and the taper mount muzzle brake, are both a few ounces lighter than the Surefire and its brake. I like using brakes with my cans to act as a sacrificial baffle. I started building a new upper for it, this time ordering an adjustable bolt carrier from Bootleg Inc. I liked the idea of adjusting the gas at the bolt carrier for three reasons. First, the gas at the BCG is much cooler and at a lower pressure than at the gas block. As the hot gasses erode at the adjustable gas blocks metering screw, it can allow the rifle to get more over gassed as time goes on. Eventually it must be replaced. Second, the heat generated at the gas block is going to effect its small parts more than at the BCG, increasing the likelihood of parts failure. Finally, the adjustment at the BCG is much easier to accomplish, so I can "go loud" with just the turn of a cartridge rim. I decided to stick with SLR for the gas block, this time their non-adjustable one.
The SLR GB6 Micro is a simple affair. Like the Sentry6, it is has a Melonite QPQ finish, lightening cuts, and a started pinning hole. It installed just like the Sentry6 with no drama. You can view the install here:
Dimple and Pin an SLR Gas Block
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| SLR .625" Gas Block Jig, Gas Port Locating Screw, Dimple Bushing |
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Position Locating Screw Over Gas Port
Mine slid on easily. |
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| Lightly tighten Locating Screw into Gas Port |
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Drilling Rear Dimple w/ 5/32" Cobalt Bit
McMaster-Carr Part# 3069A18 |
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Drilling Front Dimple
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