doublegunshop.com - home Forums General Discussion DoubleGun BBS @ doublegunshop.com Measuring barrel wall thickness

I'm an engineer or was, but the answer is simple school boy math. One draws a line to the center of each circle. Then at the tangent point, where the lines intersect the arc, draw lines perpendicular to the line connecting the centers. The distance between the two parallel lines is the distance between the arcs in question. As for arced, irregular surfaces any three points can be projected with tangent points to get the center of any arc in question. The key words here are perpendicular, parallel and straight line relationships. These require fixed points and fixed relations preferably ones at 90 degrees.

The Manson or tuning fork device method of one chap holding it on a string while the other chap rotates the gun barrels in a circle. Might provide the accuracy for two monkeys to peel an apple, but is not the stuff of accurate measurement.

The management threw us underlings a barbecue dinner last night. Seems we who are number 37 in Library size are continuously ranked in the top five in volume of sales and profit. There are some changes in the works planned to take us to the top. This means moving lots of guns and the pitch is simple - to the customer selling his gun, he's told - This what I can sell it for, this is the margin I need. The boss's rule, we don't make killings, just margins. We need guns to move them.- Why put out $20,000 buying a prestige gun that will take a year to sell when you can buy 10 guns at $200 each and turn that money over a couple dozen times in a year.

It's fun, makes sense and I like it.

Pooch,

Here's how much ground I'll give you in this debate. The manson guage is mediocre. If you had a gun in your hands that was being sold for $20,000, and it was YOUR $20,000 on the line, you can follow the instructions provided over the course of say an hour and properly blueprint the entire pair of barrels. No one working retail at Cabelas with the pressure of customers, noise, and the fact that it is other people's money is going to make such measurements feasible with a manson gauge.

The classic gauge used throughout Europe's fine gun rooms (a.k.a. a Galazan gauge in this country since they sell them) works very, very well. I can get accurate measurements within a minute or two. For a methodical blueprinting of an entire pair of barrels, I can do it in 15 minutes or less.

Other people rave about the Hosford gauge and I've seen a very well respected gunsmith outworker to Purdey's use a Hosford very proficiently...It's just not for me.

Before I ever started buying best guns from private parties I bought myself a quality gauge so I know exactly what I'm dealing with. Call me anal, I just don't have big cash to throw around making measurement mistakes on guns I buy for my hobby. Thousandths matter significantly as to value and I do tire of some of my guns over time and may wish to sell or trade them in the future. Neither buyer nor seller deserve a lemon that is misrepresented.

As to Cabelas gun purchasing strategy, I'd call it the hall of rank amateurs. (minus perhaps Dudley up in Maine) I decided to unload a few guns and brought 12 in for evaluation. They offered me $1200 for a $5000 gun wholesale. (no deal) They offered me $1800 for a gun worth $12,000 wholesale, $20,000 retail. (no deal) They then offered me $2700 for a gun worth $2700 retail. (sold) They offered me $2500 for a gun worth $2500 retail, $1200 wholesale. (sold) They offered me a $1700 for a gun worth $1700 retail. (sold)

The pricing system is a bit Zaney for me and definitely erratic. You certainly have to be able to control emotions as you hear the offers because the pricing is so very erratic. Knowledge is shallow as are the appraisals. Much easier to sell Cabelas cheap guns for full retail than best guns for 1/4 retail. A gauge would go far to help out the outfit if they knew how to price guns based on whether in proof and with thick walls.

When it comes to fine guns we, at my Cabelas, are amateurs. We are being successful at our store because we realize that fact and understand our markets. We sell to ordinary folks and strive to provide them a decent gun at a decent price. Most of the folks working the Library were buyers at other stores so they know their limitations. Cabelas is a big box store, the stuffed animals, the fancy gun library are just tools to make ordinary folks feel special. If we make our customers feel special they come back and they buy things. It's not about being a gun expert it's about maintaining a consistent profit margin, and return customers.

WE don't deal in fine guns, we have a few nice ones, but mostly we just sell guns to be used. My contention is I can get better indication of barrel wall thickness with a caliber and a good bore gauge then I can with a Manson. There are lots of indications of a thin barrel wall. Dents, the rings in the bore, the feel of the gun. These aren't the accuracy required to deal in fine guns but they are a good indication of when it's time to run.

buzz, If they are that big, then you can see them and they will be dark (deep shadow) and yes you will feel the tip of the needle passing over them, but measureable......maybe a few thousandths.

The only difference in the Hosford gage is that you use it horizontal, as to being better... Mine is too heavy to be carrying around to different tables. If the seller does not have the wall thichkness written down showing this gun, and does not give you a few days to do so, then you are at his mercy.

As eightbore showed in his use of the gage, you really need two people to use the Hosford gage accurately.
Mine like the Galazsn one, I can rotate the barrels at the same spot and read from top rib to bottom rib and then move the cone and do the same and then flip it over and do the muzzle end.

If its all about the profit margin and return customers you should not accept consignments or purchase guns for resale that require an attention to details that you are incapable of dealing with.

How do you attach the tension spring on the interior leg? I have been thinking a tension spring and a method of making the barrel wall perpendicular to the gauge would be a way to turn that Manson into a real instrument.

"Tuning fork" gauge with the Gaddy tension spring opposite the anvil on the interior leg and suspended vertically DOES produce repeatable measurements. Depth of a pit cannot be "measured" but the thickness of barrel wall (including some oxides to be sure)in the pit can be with an indicator tip that will enter rather than bridge the pit. Wall thickness adjacent to the pit can also be measured. Obviously "depth" of the pit can be extrapolated: WT adjacent - WT @ pit = Depth of pit. You have to be cack-brained to believe that the indicator can't "read" the presence of a declivity/irregularity/anomaly in the barrel wall! Obviously, whether the depth of a pit can be determined depends greatly on the size of the pit and very shallow pitting is usually detected only as physical chatter or vibration in the instrument and the indicator needle. I have yet to read the New Gospel according to the Parker lads but should be interesting.

jack

Correction to above: "indicator tip" should be "anvil on interior rod." With a bearing ball of any appreciable size as the anvil, Pooch and others are right that you can't get the anvil into a very narrow pit. You certainly can get a 1/8-3/16" diameter bearing ball to drop into a larger area of pitting and you can extrapolate the depth. The idea that a "needlelike" anvil can be calibrated to a ball-end indicator tip is no more ridiculous than the opposite but I have to admit that is not how my gauge is constructed. My face is a bit red for confusing which leg of the caliper goes in the hole! One should make sure one is not waxing geriatric before waxing theoretical.

Sorry pooch, I wanted to add to post, double-posted, deleted the first and then found your response. Few yrs. back, there were several threads on this subject which discussed Oscar Gaddy's [now discredited?] piano wire tension spring. They may still be archived or perhaps the langoliers have done their work. A number of us followed Gaddy's lead by modifying commercial guages or scratchbuilding our own to include the spring. Look at the photo of JDW's "table" gauge. The brass rod pivoting in a slot cut in the interior rod and no question hiding a coil spring behind is the same principle for the same purpose with a different execution. I'll see if I can find a picture and will post if I can.

jack

Here is a pictue showing how tension is put on the interior wall of the barrel. Opposite of that is the ball bearing. The rod for the 10 ga and 12 ga is 5/8"=.625 the ball bearing is 5/16"=.3125. The spring loaded brass bar is tapered and so is the slot. When compressed fully it has to be the same size as the shaft, 5/8" and the side that the bearing is on has to be drilled to less than half the diameter. As it is I cannot measure muzzle end barrel wall thickness on very tight choked 12 ga. guns, say .690 and less because of this.
The tapered brass cone that slides on the shaft and held with the thumb screw centers the barrel. With the breech end down it does the same.
Since the bore of the barrel is not concentric to the outside, you will always get a higher reading on one side than the other, but still you will get an honest wall thickness reading.

Here is a picture of the spring loaded brass and the dial needle resting on the ball bearing.


These gages are fairly easy to make, if I had a milling machine it would be a cinch to make these. The hardest thing is making the tapered slot and I did it on a 20" drill press with an XY table feeding .002-.003 at the most each pass. It still shook sometimes like a dog shi-ting razor blades.