doublegunshop.com - home Forums General Discussion DoubleGun BBS @ doublegunshop.com help me I think I am stupid

I was measuring a barrel for thickness.I was able to measure the inside at a marked location eight inches into the barrel of a 12 gauge and came up with .735 on the inside and used micromiter to measure the outside and came up with .850 ok so .850 - .735 = .115 I divide it by 2 and come up with .0575
Is that right.I hear you say .020 is too thin. Am I doing something wrong.
Is this ok?
Monty

Would that be 8" from the breech end? Your figuring is correct.

Are you measuring from the muzzle or the breech? Surely from the breech. The .020" minimum would be toward the muzzle I would think.

Either way .058" (.0575") is plenty.

Of course the bore may not be centered in the barrel so one wall could be thinner than the other. A wall thickness gauge is the only way to know for sure.

But you probably have plenty of wall thickness.

What gun are you measuring?

Best,

Mike

Yes I am measureing from the breech. I was exspecting it to be closer to .030 but I measured several guns including a modern perazzi and came up with around the same figure.The figures above are of a Damascus Cogwell Harrison and I just assumed the thickness would be thinner on a double gun.I'm glad to know one that I figureing it correctly and two that they are that thick.
Monty

Hi Monty,

You're definitely not stupid, just new to measuring barrels. There are several measurements that matter and you're just confusing them.

When you buy a gun, the seller typically publishes 3 measurements.

1. Choke Constriction (measuring bore diameter and muzzle diamater, subtracting the larger number from the smaller)
2. Minimum Wall Thickness (measuring the entire length of the gun's walls for the thinnest point)
3. Bore Diameter (measuring the nominal bore diameter to verify the gun is still in proof, e.g. .729 = 12 bore, .719 = 11 bore, .740 = 13 bore)

The confusing part is that minimum wall thickness is arguably less important than the measurement that is rarely published. The key measurement is 9" from the breech, does the wall thickness approximate .050" or greater? (the location where barrels like to explode and remove your hand in the process)

As indicated by other responses to your note, your measurement math is complex (but correct), yet it will mislead you. Barrels are usually bored cockeyed so that one side of the bore will be significantly different from the other side. You can't measure the outside at 9" with one tool, measure the inside at the same 9" with another tool, do some math and then get a true answer. Your results are misleading and do not account for concentricity issues of the barrel.

You need a $500 wall thickness tool to give you the actual measurements you truly need with any amount of reliable accuracy.

Imagine this scenario: There is a dent that was raised on your gun 12" from the breech of your gun and it was then honed from the inside and polished from the outside. It is invisible to your eyes from any location. Any tool short of a wall thickness gauge will omit that reading that actually shows the gun is only .014" in an area that is otherwise .038" at the same length from breech, but just at a different location along the circumference of the bore. Dangerous!

-Rookhawk

I will add my 2 cents to this.

A $100 wallthickness gauge will do the job and is available at Brownells. You should modify it with a little spring clip to aid the reliability of the measurements as many here have done.

Most of the old gun barrels I've examined seem to have a double taper from the breach. The first taper is steep for about half the overall taper, then the second taper is shallower and goes down to the straight section of the barrel O.D.. From there on, the O.D. is constant diameter.

Barrels are manufactured by boring a larger diameter bar with a "gun drill" which is a single sided cutting tip to make it cut a long straight hole (believe it or not). The bore is then reamed and and honed to finish diameter. The bore is straight within a very close tolerance at this point. Then the O.D. is machined concentric to the I.D., sometimes over a mandrel. This results in a barrel that has the tapers and straight sections concentric to the bore within a few thousandths. When the barrels are soldered together and filed/polished is when the largest variation in wallthickness is most often introduced. Variation is furthered during any refinishing on older guns. You can often see it at the muzzles of a double where the outside wall is thinner than the area between the ribs.

I will add that a well raised dent that got honed and the outside polished should not have any thinner wall than the adjacent area. For the wall to be thinner, the outside would have to be "dished" by improper polishing. It does occur, but dent raising and refinishing doesn't have to result in this condition.

What Hawk said but you can get a wall thickness gauge like I use for $200.

By reading the hyroglyphs on the barrel flats you can sometimes tell what bore diameter range was when the barrels left the factory. And by comparing them to their current bore diameter you may find they are original diameter. That is why I asked what gun they were from.

Of course the American made guns do not have this information stamped on their barrels.

Best,

Mike

Chuck H;
Excellant post, I will however add one little bit. This is not universal but I have observed (& measured) a good number of bbls which had a slight swelling of the OD at the choke area. Thus if you mic the OD at the muzle & then again about 6" back it is not at all uncommon to find the muzzle dimension to be several thousandths larger than the 6" back measurement.
A lot of guns will measure exactly as you said but quite a few as well will have this enlargement a of the choke area.

the barrel flats also have a 13 stamped on it along with not for ball and a 14m.
Monty

I am confused on guns it's a W C Scott not that it makes any difference.It has a little choke in the second barrel and gets wider on the first barrel. I have never heard of a choke getting wider. Whats that all about?