jds holler
Gold $$ Contributor
No stupid question, right. I hear this term all the time, and it occurs to me that I don't really know what it involves, or how it is done. Could someone please enlighten me. jd
Please Explain Air Gauged Rifle Barrels
- Thread starter jds holler
- Start date
Toolbreaker
Gold $$ Contributor
It's a means of measuring the dimensional accuracy of a bore. As calibrated to a given standard, the gauge "leaks" X amount of air. When in use, if the gauge leaks more air than it's calibrated to, it registers the measured bore larger than the standard it was set to. If it leaks less, then smaller. Here's a diagram of a gauge head inside a bore.
With what you're asking about, a barrel maker would insert the gauge into the bore, then slowly withdraw the gauge along the length of the barrel. As he was doing so, he would observe an output from the gauge (analog or digital) and check for inconsistencies of the bore size along the length. I personally don't use them, but I'm pretty sure there are a few on here who regularly do
. I'm betting they can fill in the details.
With what you're asking about, a barrel maker would insert the gauge into the bore, then slowly withdraw the gauge along the length of the barrel. As he was doing so, he would observe an output from the gauge (analog or digital) and check for inconsistencies of the bore size along the length. I personally don't use them, but I'm pretty sure there are a few on here who regularly do
. I'm betting they can fill in the details.
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I would say that not too many people use one...not too many can afford it or have the need unless you manufacture barrels. The significance of the air gauge is that it will readily read if the barrel is oversize. Prior to the advent of the air gauge barrels were "checked" with what was known as a star gauge. This was a hardened steel tool that had "stars" very carefully ground on each end to match the intended bore size. It was a certain length and also very carefully ground to be dead straight. It was inserted into the bore and if it stopped then the bore was either undersized or not straight. It kind of worked by feel so it wasn't as positive as the air gauge at letting you know if the bore was oversize and/or oversize and not straight.
It was accurate though if used carefully and would tell if the barrel was on size and straight. If I remember right the biggest problem was, like the single point cut rifling also of it's day, it was just slow all the way around. Slow to produce an accurate star gauge and slow to use. It also would wear out. The air gauge method solved most if not all of the problems. A barrel passing either test method is still no guarantee said barrel will shoot.
It was accurate though if used carefully and would tell if the barrel was on size and straight. If I remember right the biggest problem was, like the single point cut rifling also of it's day, it was just slow all the way around. Slow to produce an accurate star gauge and slow to use. It also would wear out. The air gauge method solved most if not all of the problems. A barrel passing either test method is still no guarantee said barrel will shoot.
Both of you gentleman are pretty dead on in your equation.The uniformity from one end to the other is very important that tells me the heat of steel we're using is very uniform in shrinkage.
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jds holler
Gold $$ Contributor
Thank you gentlemen, your replies give me a clue.
My mind now pictures something like an undersized rifling button with measured air flow provided.jd
My mind now pictures something like an undersized rifling button with measured air flow provided.jd
Yes sir, and this is a very important point. It's kinda neat and fun to look back and see how things evolve. Prior to the advent of the "star" gauge the method was simply seeing if a length of straight ground hardened steel rod would pass thru the bore. This was just thought to be a little better indication of straightness compared to looking thru the bore at a light {Remington??}, but only if the bore was dead on size...if it was over and bent the rod could possibly still pass.
The most info I have seen regarding the star gauge process and then air gauging was in one of Brophy's books...can't remember which one, but he did a pretty good job of explaining it and I think he even had a photo of an old star gauge from Springfield Arsenal. I'm glad I don't have to make them things for a living!!!!
My mind pictures not a GD one of the current factory barrels made passing the test!!!!!
{Except Tikka}
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jds holler
Gold $$ Contributor
Seems to me that a straightness check with a precision rod as well as an air gauge check would be good; and worth a little more money to put it into the "premium barrel" category. jd
Well.....the problem is that when you see a barrel advertised as "air gauged" you still don't know how much air pressure variance they allowed...some, none, how much is really too much???? I have seen several barrels advertised this way, but never saw any published QC parameters. It reminds me of Kimber's "MOA guarantee", read it if you get a chance...talk about bull schidtt!!! it's not worth the paper it's written on!!!!
When I checked for air guage tolerance I always grade for straightness in case something strange happened in the stress relief process I have looked threw several thousands of barrels over the years a trained eye know what to look for is an art in itself.
Watched a 'smith at the USN match conditioning shop air gauge a bunch of new broach rifled 7.62 chambered Garand barrels. All had .0002" or less spread but average groove diameter ranged from .3085" down to .3077". All gauging .3079" or less were set aside for match conditioned rifles.
As I remember, the Sheffield 30 caliber air gauge used was graduated in .0001" steps.
https://willrich.com/product/gun-barrel-air-gages/
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butchlambert
Site $$ Sponsor
I've seen some record breaking barrels that did not have a straight bore.
BB, where did you get your info on the gain twist barrels or you really don't know? Just trying to interpret your post.
BB, where did you get your info on the gain twist barrels or you really don't know? Just trying to interpret your post.
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