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I Should Know This But I Don't. (Read 767 times)

burntwater Oldtimer Offline Posts: 579 Location: Michigan Joined: Feb 5^th, 2018	I Should Know This But I Don't. Feb 8^th, 2026 at 4:05pm	Print Post
	Anybody want to explain to me what and how "Air Gauging" a barrel works and how accurate is it ?I've seen this forever with Douglas barrels but haven't a clue Rick


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bpjack Frequent Elocutionist Offline Posts: 3668 Location: East Olympia, WA Joined: May 18^th, 2008	Re: I Should Know This But I Don't. Reply #1 - Feb 8^th, 2026 at 5:00pm	Print Post
	I didn't know either so I asked Mr Google. (You need to *Login* or *Register* to view media files and links)
	« Last Edit: Feb 8^th, 2026 at 5:17pm by bpjack »
	ASSRA # 11318 Experience is something you don't get until just after you need it.
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ssdave Frequent Elocutionist Offline Posts: 2115 Location: Eastern Oregon Joined: Apr 16^th, 2004	Re: I Should Know This But I Don't. Reply #2 - Feb 8^th, 2026 at 6:06pm	Print Post
	For those that don't want to watch youtube, air gauging works by putting a button slightly smaller than barrel diameter into the barrel, and introducing presurized air behind it, while monitoring the pressure. The button is moved down the barrel, and the air pressure changes are graphed vs button location in the barrel. As the barrel diameter changes, the leakage around the button causes pressure changes. The system has been calibrated for button size vs bore size vs pressure to indicate variatons in diameter. Barrels with the least variations in pressure and thus diameter are more consistent performers. Douglas has established a pass/fail criteria for it's air gauge barrels to set the level of quality.


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boats Frequent Elocutionist Offline Posts: 7721 Location: Virginia Joined: Apr 23^rd, 2004	Re: I Should Know This But I Don't. Reply #3 - Feb 8^th, 2026 at 7:49pm	Print Post
	It’s one of the reasons Douglas produces top quality barrels in volume at reasonable prices. Boats


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gunlaker Frequent Elocutionist Offline Posts: 2560 Location: lower mainland, B.C. Joined: Dec 13^th, 2010	Re: I Should Know This But I Don't. Reply #4 - Feb 8^th, 2026 at 7:51pm	Print Post
	I always wondered about that. I have a Green Mountain barrel, which I heard are air gauged as part of the process, but it has a rifling deformity that is huge. When you push a tight patch down it, it feels like a big bump when you hit the really bad patch. It is also full of loose spots which are easy to feel when slugging it. Shouldn't air gauging have caught that? Or is air gauging done through the bore before rifling it? Chris.


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Smoke Oldtimer Offline Posts: 834 Location: San Diego Joined: Feb 24^th, 2013	Re: I Should Know This But I Don't. Reply #5 - Feb 8^th, 2026 at 10:26pm	Print Post
	I have a Douglas barrlel with the same kind of "push a patch thru" bump. A bore scope showed a deep gouge in the rifling, which doesn't seem to stop it from shooting .5 MOA groups at 100 yards when I do my part. I suspect that the air guage just shows that the bore and rifling are mostly consistent size within some standard, from end to end. A soft lead bullet fired and captured so that one could look at the output end of a shot might more useful.

	The longer everything goes according to plan, the bigger the impending disaster - Howard Taylor
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boats Frequent Elocutionist Offline Posts: 7721 Location: Virginia Joined: Apr 23^rd, 2004	Re: I Should Know This But I Don't. Reply #6 - Feb 8^th, 2026 at 10:46pm	Print Post
	The air gauge is just a tool. What’s done with the information is up to the barrel maker. And it’s possible makers overlook variances by mistake Owned at least a dozen Douglas barrels, 8 on CPA’s others on High power Silhouette rifles, never felt bumps or hard spots. If I did would return it. Boats
	« Last Edit: Feb 9^th, 2026 at 7:18am by boats »

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gunlaker Frequent Elocutionist Offline Posts: 2560 Location: lower mainland, B.C. Joined: Dec 13^th, 2010	Re: I Should Know This But I Don't. Reply #7 - Feb 8^th, 2026 at 11:28pm	Print Post
	boats wrote on Feb 8^th, 2026 at 10:46pm: The air gauge is just a tool. What’s done with the information is up to the barrel maker. And it’s possible makers overlook variances by mistake Owned at least a Douglas barrels, 8 on CPA’s others on High power Silhouette rifles, never felt bumps or hard spots. If I did would return it. Boats I like the Douglas XX barrels too. This one particular GM barrel is the only barrel I've ever had that had a bump in it like that (from new). It seems to me though, that if this barrel passed a test with an air gauge, then whoever was doing the testing must have not been good at the job . Chris.


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bohemianway Oldtimer Offline Posts: 704 Location: Andover, MN Joined: Apr 16^th, 2004	Re: I Should Know This But I Don't. Reply #8 - Feb 9^th, 2026 at 8:58am	Print Post
	We used to demonstrate air gauging in engineering school labs. My recollection is that the gage is fed with a regulated pressure and a flow meter is used to monitor the leakage rate past the probe. It is not a very fast response type measuring tool so if you move to fast it will miss slight imperfections. It is more of a general bore size consistency tool. The saddest part of all this is that the school had tons of gauges and tooling from donations from Rock Island Arsenal and most of it was tossed out in the 1980's and the wooden boxes used for gun and accessory cases. How I know is because I still have some of the boxes. Charles


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gunlaker Frequent Elocutionist Offline Posts: 2560 Location: lower mainland, B.C. Joined: Dec 13^th, 2010	Re: I Should Know This But I Don't. Reply #9 - Feb 9^th, 2026 at 9:15am	Print Post
	Interesting! Thanks.


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burntwater Oldtimer Offline Posts: 579 Location: Michigan Joined: Feb 5^th, 2018	Re: I Should Know This But I Don't. Reply #10 - Feb 9^th, 2026 at 9:49am	Print Post
	So thanks gentlemen for the explanation though it seems to me that there must be a bit more to it. I tried to find a video of the procedure but couldn't. Rick


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GT Frequent Elocutionist Online Posts: 2337 Location: Northeast Wyoming Joined: Jun 28^th, 2015	Re: I Should Know This But I Don't. Reply #11 - Feb 9^th, 2026 at 10:15am	Print Post
	Charles is correct in the speed a gage is pushed down the bore, even then it can give false info. While in engineering in my younger days, the non destructive test lab had one we used a bit. As students we learned that there was a lot going on in a bore, not as perfect as we were led to believe. Being ten years senior to most of my fellow students and having worked in the trades, I was a skeptic on the accuracy, with reason. I picked up an air gage system at an auction in the distant past thinking it may be a good addition to inspecting barrels. I made a couple of gages, experimented with several ideas for gages, and if everything is just perfect and you take your time you will see a lot, without ever looking down the bore. For the hassel of setting things up, turning a half dozen lead slugs and slugging a barrel is a far easier op. With a slight amount of practice, it will tell you as much, if not more. I've only had a bore scope for the last 12 years or so and the view of the bore only gives a part of the picture. My 2¢ Greg

	"To invent, you need a good imagination and a pile of junk" T. A. Edison "The trouble ain't that there is too many fools, but that the lightning ain't distributed right" M.T.
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jhm Frequent Elocutionist Offline Posts: 1989 Location: georgia Joined: Sep 4^th, 2011	Re: I Should Know This But I Don't. Reply #12 - Feb 9^th, 2026 at 10:31am	Print Post
	It is used a lot in the aircraft industry where bore sizes are critical. I used air gauges as my primary gauging at my last employ. My general working tolerance was eighty millionths to a tenth and twenty millionths on the valves I built. A great tool in any industry if you need accuracy to the highest degree. It will show variances even to the tiniest degree depending on how sensitive the instrument is. You can set them to whatever you need. JMH


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burntwater Oldtimer Offline Posts: 579 Location: Michigan Joined: Feb 5^th, 2018	Re: I Should Know This But I Don't. Reply #13 - Feb 9^th, 2026 at 3:47pm	Print Post
	Very interesting. I have to ask when people talk about 'air gauged ' barrels it seems the process is anlways connected to and advertised by Douglas. Do other barrel makers use this equipment ? Rick


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jhm Frequent Elocutionist Offline Posts: 1989 Location: georgia Joined: Sep 4^th, 2011	Re: I Should Know This But I Don't. Reply #14 - Feb 9^th, 2026 at 4:27pm	Print Post
	I am sure they do. Air gauging is an industry standard for accuracy in measuring bores of all kinds and not just the barrel manufacturing industries. JMH


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