- November 20, 2018 at 4:56 pm #68990GoodsteelKeymaster
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This is something that has bugged me for years. If I’m turning an AR-15 barrel, I have simply duplicated the gas block journal location from another barrel because the information on where to cut it is practically non-existant online, and I don’t know where else to look. Probably buried in a book somewhere, but I wanted a way to tell by simply going by the gas tube and block I have in front of me.
Who knows? Maybe the gas tube manufacturer had a brain fart and made it too long, etc etc. I want to be able to figure it out every time with the actual parts in front of me, without having to measure another barrel, and without having to cross my fingers and hope what I cut actually works with the gas tube in hand etc etc etc.
Well I scoured the internet and made a compiled list of every gas journal location claim made and wrote them down. This was very frustrating because all it showed was that NOBODY has real numbers for this.
SO!!!! I tore apart an AR-15, lined up gas tubes from all four of the main sizes, and I carefully measured them. I also measured the BCG of the AR and compared measurements between several BCG’s I had on hand. I was interested in the depth of the hole in the key, and the depth that the common barrels I had on hand located the tip of the gas tube when installed on the barrel. What I found was that the predominant location of the gas tube tip is .125 from bottoming out in the key hole.
Given this fact, the gas block journal must be located a certain distance from the tip of the gas tube, and this in turn locates the tennon shoulder, and the DISTANCE BETWEEN THEM IS THE INFORMATION I WANT.
Stated another way, I wanted to be able to determine the exact distance from the front of the barrel extension rim to the shoulder of the gas block journal.
After hours of careful measuring and extensive comparisons, I have found a method that gives this exact information every time.
1. The gas tube is assembled with the gas block.
2. Measure the distance from the rear face of the gas block to the tip of the gas tube.
3. Subtract 2.175 inches from this measurement.
4. The resulting number is an extremely common distance used in the industry and will be very close to locating the tip of the gas tube 1/8″ away from bottoming out in the gas key.
Near as I could tell with the examples I have in front of me, this measurement carries a tolerance of +-.025 inches.
If I find more information on this subject, I’ll post it, but for the moment, this can be used at least in part as a sanity check for your tenon and journal cut locations.
Again: Gas tube length from gas block, minus 2.175= distance between tenon and journal shoulders plus or minus .025.
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