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+--- Thread: RCA Adjustable Gas Key works as advertised (/showthread.php?tid=9100)
RCA Adjustable Gas Key works as advertised - NugginFutz - 03-07-2015
Made a trip to the range, today, to evaluate the Rubber City Armory Adjustable Gas Key. There has been some speculation regarding the use of these as having the potential for increased cam pin wear or even bolt lug breakage, so I thought I'd document my experience.
The Naughty gun: A 5.56 AR with 22" tube, which loves to throw brass a good 12-15 feet between 2 o'clock and 3 o'clock. This is a RLGS BHW barrel, with a 1:8 twist.
I picked up the gas key a couple weeks ago, and a new stripped bolt carrier from DPMS, last week. Last night, I assembled the key and carrier, using LT Red to seal the mating surfaces and used the factory supplied screws with factory applied thread lock. Torqued to 40 "lbs, but did not stake, yet. (I'll get to that, when I find someone local with a staking jig.)
The accompanying video shows bolt travel and performance at various gas settings.
Video from range:
![[Image: 38318295036_5d661b4847_o.jpg]](https://farm5.staticflickr.com/4516/38318295036_5d661b4847_o.jpg)
Observations are as follows:
Interestingly (but not unexpectedly), when the screw is fully closed, there is definitely some movement, but not very noticeable until the video was slowed down. I estimate the carrier travelled .3 inches, based on the number of teeth on the carrier moving into the receiver. This means that the bolt had definitely rotated mostly, if not completely, to the unlocked position. It did NOT, however, appear to begin case extraction. I suspect this was due to insufficient force to either finish unlocking the bolt or overcome the friction of the expanded cases. No extractor claw marks on case rims or other signs of distress.
As I backed out the screw, it was clear that it didn't take much before the bolt's gas rings began to dominate the extraction process, again. At 1/2 turn, I didn't see any change, but at one full turn, I was getting full extraction but without lock back.
At a one and a half turns, the bolt was cycling quite effectively, as well as locking back on an empty magazine.
The payout:
Ok - so what did this get me? Well, It definitely did what I had hope it would, and that was to tame ejection. Brass is now traveling about 6 feet, and landing at 3-4 o'clock.
I didn't originally intend to go this route, but the concept of an adjustable gas key was intriguing. Could I have used a conventional Adjustable Gas Block for this? Certainly, but it would have cost at least twice as much, if not significantly more, since this rig would have needed a very low profile block.
The other major benefit I can see, though, is this: Anyone with a pinned/welded muzzle device might be married to their existing gas block, but they now could have something besides buffers and springs to adjust when tackling cycling issues.
Perceived downsides:
However minor the carrier motion was at full cutoff, it will always be in the back of my mind. Each time I break down this upper for cleaning, you can bet I'll be watching for signs of wear on the cam pin and cam journal. On the other hand, I don't think I'll see this 5.56 bolt sheer any lugs. As noted, the case rims show no signs of their little heads being torn off, so I'll relax a little on that.
Secondly, unless you order a new carrier as I did, you will need to remove they carrier key from an existing donor, and that can be a real nut buster. I gave one of mine a half-hearted attempt, but lost my nerve when the socket driver was showing visible deflection with no sign of the screw breaking loose. Having never taken one apart before, I didn't know if it was almost ready to come loose or shear, so I took the cowards way out. Heck - by my way of thinking, you can't have enough spare gun parts, anyway.
So, until and unless I start seeing any signs of wear, the RCA adj. gas key will go back into this rifle as soon as I get it properly staked. It's proven itself well enough for me.
Thanks for reading.
RCA Adjustable Gas Key works as advertised - Younger - 03-07-2015
Thanks for the great info. I do have a pinned 14.5" Grendel that is quite energetic when extracting the empty cartridge.
Does anyone have any info on the Sun Devil adjustable key?
RCA Adjustable Gas Key works as advertised - montana - 03-07-2015
Great video and report. I would have guessed the bolt carrier would have had more rearward movement with the gas key screwed closed. Cam pin wear wouldn't worry me at all after watching the little rearward movement created with the adj gas screw screwed shut. With the screw opened to optimal setting the cam pin stress would be nil. I'm still a little concerned if the adj gas key screw wont unscrew during recoil . Only time and use will tell. I would heat the gas key screws if they showed resistance when trying to remove them. They could have had red loctite put on them. Solving the pinned flash hider problem was the main reason I was looking at the adj gas key solution. Keep us informed on any changes. Thanks again for your report.
RCA Adjustable Gas Key works as advertised - NugginFutz - 03-07-2015
Thanks, guys.
Montana - This key uses a double set screw configuration. Both have a heat resistant thread lock on them and all you do is find the Happy Place, and lock it with the second set screw. Again - both are kept secure with thread lock.
As far as how much motion on a fully shut down setting goes, I worked out that the Gas Key has about 1:8.7 the cylindrical surface area as the bolt. Assuming worst case 52kpsi, it comes out to about 1,300 lbs peak pressure at the key versus 11,500 lbs peak pressure at the bolt. So, relatively speaking, there is really very little pressure being applied on just the gas key. Hardly enough to get things exciting, it seems.
Once the screw is opened, the amount of pressure at the key goes down dramatically. I doubt that there is more than a few hundred pounds peak pressure on the key, now.
RCA Adjustable Gas Key works as advertised - montana - 03-07-2015
Thanks for the update on the double set screw. Your report is what I have been searching for. Thanks again.
RCA Adjustable Gas Key works as advertised - spectra - 03-07-2015
I am in the middle of collecting parts and this is on the list. I think for 125 or so I can get a BCG from RCA with the AGK.
I think after seeing this I am ordering this weekend.
RCA Adjustable Gas Key works as advertised - AZ. Old Guy - 03-07-2015
Younger Wrote:Thanks for the great info. I do have a pinned 14.5" Grendel that is quite energetic when extracting the empty cartridge.
Does anyone have any info on the Sun Devil adjustable key?
I have the Sun Devil adjustable key in mine but I haven't messed with it yet. Call Dave at Sun Devil and he can fill you in. Great guy.
RCA Adjustable Gas Key works as advertised - LRRPF52 - 03-07-2015
Excellent report and video. I have one waiting to go on a stripped carrier as well. I have the staking tool also. Just send me your BCG and I'll hook it up.
RCA Adjustable Gas Key works as advertised - Bwild97 - 03-07-2015
NugginFutz
Quote:However minor the carrier motion was at full cutoff, it will always be in the back of my mind. Each time I break down this upper for cleaning, you can bet I'll be watching for signs of wear on the cam pin and cam journal. On the other hand, I don't think I'll see this 5.56 bolt sheer any lugs. As noted, the case rims show no signs of their little heads being torn off, so I'll relax a little on that.
You might want to watch the back side of your bolt lugs as well.