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What is "Air-Gauging"?
fishkiller41
Member Posts: 50,608
How is it done and what is the purpose of "Air-Gauging a rifle??
Jeff
Jeff
Comments
Any variance of pressure..high or low..indicate either a tight or a loose spot in the bore.
A precision ground plug just slightly smaller than the bore of a rifled barrel is placed in a rifled barrel and air is forced past the plug and measured as to how evenly the pressurized air flows past the plug as it is moved down the bore line. The gauge shows the variances between tight and loose spots in the barrel. It's merely measuring how consistant or inconsistant the bore/groove diameters are over the length of the barrel. The advertising folks tied this aspect of gauging into accuracy and to a certain extent a consistant size bore/groove should yield better accuracy than one that is frought with inconsistant diameters.
It doesn't get as much mention as it once did, although as I understand it, most of the factory or larger barrel makers still air gauge.
Best.
Jeff
Do they scrap a barrel at +.0005/-.0001 from nominal or do they accept +.010/-.001 as being in spec. IOW saying you measure each barrel with a air gauge (or charge extra for one that has been gauged) means nothing if the tolerance limits are unknown.
It just depends on which source you choose to reference for that information. The manufacturer's websites will generally have that information displayed with fair prominence. The problem that I encounter is the fact that the average shooters have no understanding of just what these numbers really mean and just how small most of them are. That's not a criticism at all, it's that the numbers don't have any real bearing on anything in most cases. The makers like to try to make it a big deal by adding more zeros so that they are seemingly more accurate in the manufacturing. In most shops, the gauges can't resolve dimensions that small.
The reality of the barrel making process is that the variations should be at a manufacturing minimum and most importantly, consistant from one end to the other and that the muzzle is the tighter of the two ends. The interior finish as applied to those dimensions is a big factor as well. Now that's very simplistic but I don't see any reason to embark on a discussion of the actual physics.
In addition to the specifications, barrel makers rarely 'scrap' a barrel if they can avoid it. Smaller bore/groove barrels are reamed up to the next size and started through the rifling process again. The QC aspect comes into play to attempt to determine why the error(s) occured. Everything in the process is examined from the steel to the reamers, the oil, oil pressure, speeds and feeds, and of course the button or cutter box.
Brownells does list that information basically as supplied by the manufacturers:
Douglas Ultra-Rifled barrels
"Button rifled for unparalleled smoothness with tolerances for groove diameter and straightness held to 0.0005"."
Wilson Arms standard grade M1 Garand barrels:
"Bores are held to 0.0002" maximum tolerance from throat to muzzle."
Wilson Arms 98 Mauser barrels:
"Stress relieved and machine lapped then air-gauged with a muzzle to breech tolerance of 0.0001";"
Shilen Select Match:
"Button rifled, stress relieved, hand lapped, air gauged and optically inspected to ensure that each falls within the Select Match category. That means within 0.0003" of specs and uniform over the entire bore to within 0.0001"... "
Lothar Walther:
"...that assures bore uniformity to within 0.000227,..."
Best.
My question is; are those values for total Diameter? If that is the case, isn't the tolerance 0.00005" per side (given a gauge of 1 ten thousandth of an inch)?
0.00005" is 50 millionths if an inch!!! The only way to hold that sort of tolerance is by grinding (as far as I know) I am surprised that that can be held in a cut rifled barrel.
Do you know what the Standard Deviation is on the diameter of a match grade bullet?
Best, back at you
EDIT: Watch those ZEROS. 0.0001"/2 = 0.00005" (another reason that metric might be easier. Microns vs Millionths)
Wilson & Shilen tol = 0.0001"
I am taking a GD&T refresher this coming week. I'm going to have to bring this up in class. Concentricity and Run-out over 20 to 28 inches!!! I'm not sure we hold those Tols for our Cams nor Crankshafts - I'm going to have to check the Drawings.
Not to stray to far off topic, but IMHO if your trying to make a 6mm barrel and it falls out of spec and has to be reamed out to 6.5mm than it was scrapped (at least as being a 6mm is concerned).
I don't have any issue with any of the companys you mentioned, or the tolerances they hold, but there is at least one company that pushes the idea that "air gauging" makes their barrels better, even if they do look like 6 miles of bad road on the inside.
The dimensions used can be expressed in both ways depending on what the manufacturer is trying to prove. In most cases they are stating the total variance and then as in the Lothar Walther example, the uniformity. I contend that the uniformity is the more important and meaningful of all of the information. Note that Shilen states that their products "fall with 0.0003" of specs" without defining what their specs are and then state what they expect for uniformity as a separate specification. But again, I think consistancy and uniformity is the important factor as long as the barrel is withing the performance dimensions. The closer the barrel dimensions are to the dimensions of the bullet the better the bullet and barrel should perform.
Again, so much of this is open for discussion and opinions. Some of John Krieger's barrel have smaller internal dimensions for the .308 Palma shooters using the 155 gr. Sierra bullets. The bore dimension is listed as 0.298" and the groove selection is then 0.3065", 0.307", 0.3075" where the standard dimensions are 0.300" bore and 0.308" groove. This is essentially a very minor squeeze bore which is due in large part to the design of the 155 gr. Sierra Palma bullet. Now, where do we discuss the obvious pressure factors from these dimensions?
On the other hand, I've worked with barrels that have variations in the internal dimensions that I usually would reject that seemingly performed O.K. In most of these instances I was able to find that one end of the blank had a slightly tighter dimension and I could use that one for the muzzle.
Half of the 0.0005" (five tenths) dimension is 0.00025" or 2-1/2 ten thousandths.
"Do you know what the Standard Deviation is on the diameter of a match grade bullet?"
No, sorry I don't. But again, it's the consistancy and closeness to actual dimensions that is considered the ideal. The bullet makers that I've worked closely with do have a tiny undersize or 'at size' dimension for the shank of the bullet. The shank can be a tenth or two under because there is a slight bulge at the base of a flat base bullet that is slightly groove oversize by as much as 6 tenths (0.0006") as in 0.3086" for 30 caliber bullets. This will vary maker to maker due to die design and technique.
Tailgunner1954,
I agree insofar as semantics are concerned. I was going to state that but I thought it was O.K. to leave it out. If i was making a barrel for a specific bore/groove and missed it for some reason, I consider it to be scrapped as far as that particular barrel is concerned. But I consider it to be "salvaged" and perfectly acceptable if I ream it up to the next size and it is within specification for that size and consistant. It is not sold as a "salvaged" barrel but as a good barrel for that size.
There are exceptions to everything that we do and say in this industry. Advertising writers and Ad Execs attempt to do the best for their clients even if those clients are not necessarily considered to be in the top ranks of their part of the industry. It's the marketing and selling of a client in the advertising industry. I know exactly what you're saying. We, as consumers, have to be smarter and work harder just to stay even when trying to maintain quality these days. That's one of the great benefits of these forums, the better dissemination of high quality information.
Best.
Added for iceracerx,
I know you are aware of these things but I added all of this for the sake of discussion.
Geometric Dimensioning & Tolerancing is one of those courses that I never want to repeat although I could use a refresher, I'm sure. I had to have it in conjunction with AutoCad and another course later for advanced work.
Numbers used in advertising are always suspect as far as I'm concerned. Those quoted by a manufacturer can be manipulated into another realm altogether if we don't read their information carefully. Then we have to always ask "under what conditions are these numbers being derived?" I can guarantee you that there is no barrel maker that is subjected to ISO 9000 or newer standards and I doubt that the dimensions are being measured in a high tech, controlled atmosphere QC room. Even as expensive as the air gauge equipment is, it is still the easiest method of obtaining the 'measurements' along the entire length of a barrel blank.
Personally, I can't imagine using numbers that go beyond the 4th decimal place for barrels or anything else in gunsmithing. Anything can be measured to tenths if you have the appropriate measuring tool for that type of resolution. Cut rifled barrels can be measured to tenths with the right tools and the cut is capable of being made at least that accurately as long as the steel and the original hole are as good. The depth of the cut though needs to be somewhat efficient if you have any hopes of making money while making barrels.
Buttons barrel makers have a more difficult time since the steel formula can affect the final bore/groove size. Throw in ambient air temperature, lubricant properties, rate of pull, twist rate, coefficient of friction, the imparted stress inherent to the process and maybe another dozen variables and it's a miracle that we can get a consistant hole with grooves at all. But the button barrels can be consistant once you get a set of variables somewhat under control and start a run of barrels.
The end result of these dimensions is that very rarely are they truly being measured accurately. In most cases these numbers are derived and adjusted to give the impression of accuracy. If I measure the initial hole and it's 1 tenth under or over some number, then I could say that it is 1/2 of 1 tenth per side couldn't I? The other problem that this opens up is the tools themselves. As an example, just because a set of calipers has 5 zeros after the decimal point, does not mean that I or the instrument can resolve those dimensions or that the number is usable or meaningful. The concept of significant digits takes on a whole new meaning...
Best.
So in theory, the better barrels from a standpoint are the hammer forged ones, that are forged around a mandrel of known dimensions?
I know that opens up another whole can of worms and a lot of dependant logical statements, but my point is, with a manderl/hammer forged barrel, one can almost litterally measure the rifling before the barrel is even made. I know you love this stuff, that is why I like to bring it up.
Best