doublegunshop.com - home Forums General Discussion DoubleGun BBS @ doublegunshop.com Question about bore

jOe, I have measured some L.C. Smiths that had a tapered bore, one being a Long Range gun.

Small Bore, I did that, when I put the gage into the muzzle end after being zeroed on the .700 ring. It read .718 to the full depth of 12", withdrawing it slowly read the same until it was out of muzzle. The right barrel was .718 all the way. As I stated, from the breech end they both were .728-.729 which is common for older "elsies".

Oldfarmer, I do agree with you, pattering is the only way to tell, and with different loads and different manufactures.

"I'm with the Rabbit there's been some honing going on somewhere."
This is where I'm confused, backboring or honing, doesn't that remove metal and wouldn't the dial gage read larger when it hit that area in the barrel?

An example: An 1892 Greener with the mark of 13 on the watertable means it is a 13 ga. or .710 bore. The bores now read .728 and .729, so they were most likely backbored as that is a lot of honing. Gun still has .009 choke in right barrel and .034 in left. (still have minimum .022 wall thickness). It does shoot very well with low pressure, low velocity reloads (under 7,000 psi), just have to get used to the side safety.

I wonder if this was an attempt at back boring???

One thing here stands out;
A .724" dia plug gauge will not enter a .718" dia bore!!!. You need to do some careful double checking of all your readings. Something is not checking out somewhere.

Sounds suspiciously like a partial backbore job. Examining the muzzle end for signs that the choke was relieved to cylinder bore might reveal something. Also, looking at the finish in the breach end may tell something. .732 is about what you might expect after running a Clymer .730 dia backboring reamer down the bore and polish it afterward.

Miller, that's what is confusing. I know the plug gage size is correct. I make them from aluminum and have them from .722-.732 to use in barrels that have dents in them. Once I find the right size that won't fit through to dent, I make a steel tapered plug that fits under the dent and then tap it snug under the dent and gently tap the outside until the plug gage slides through until I get the right size to bore.

The Skeet's gage with the dial coincides with the readings taken from the breech end of .728 and .729 with the aluminum plug gage of .728 that only goes to 9 7/8". Naturally I cannot get a .724 into the muzzle end and why the Skeets gage reads .718 for 12" is beyond me. I am going to make a .717 plug and try that from the muzzle end and then from the breech end. If there is clearance(light)around it I will then know it is bigger in that area, and that the Skeet's gage is doing something different, but why.

Chuck, again, if the bores were partially honed or backbored, wouldn't that give me a larger reading on the Skeet's gage? If I compress the balls to 0 on the gage and gently release them, the dial goes to +, if I compress the balls at 0 the dial goes to -.

The dial after zeroed with the setting ring of .700 went from 0 to + .018 meaning to me that the bore is smaller, if it had gone to +.046 then I would say it was backbored.
Am I making sense?

Made a .717 aluminum plug, would not go in from the muzzle end of the right barrel, inserted it in the breech and it went in to 27". Verified the readings with a telescoping gage at the muzzle and got .716 at end and .718-.719 in as far as I could reach and tighten the stem.

I'm begining to wonder if these gun was special bored.

Choke constriction is measured from the bore diameter IMMEDIATELY before the shot enters the constriction and at the smallest diameter of the constriction. Parallels and divergents are not considered in measuring constriction. 0.729 at constriction entry and 0.700 at the smallest diameter of the constriction is a 0.029 constriction.

As I read your description, for the right, the gage read 0.718 from a full 12" insertion to the muzzle; that would be a cyl constriction. For the left, the gage read from 0.719 to 0.710 for 0.009 constriction; a IC constrictiion.

If there ever was a gun with a truly, continuous taper from chamber to muzzle, the effect would be as no choke(cylinder). If there was a continuous taper from chamber to the beginning of the choke taper, only the last few inches of bore, leading up to the choke taper, would matter.

Thanks again Jim and Rocketman. I understand all of that. The thing I don't understand is few have said gun was either honed or backbored. My reasoning is that if that were so, the bores would be larger not smaller.

This is what I have come up with since the last post. Using plug gages, right barrel from breech; Left barrel from breech:
.727 in 5" .727 in 10"
.726 in 9" .726 in 24"
.724 in 22 15/16" .724 in 24 15 /16"
.722 in 25" .722 in 25"
.717 in 27 1/16" .717 in 25 7/8"

Wall thickness (using gage hanging from ceiling and rotating barrel from top rib to bottom rib.
right barrel left barrel
9" .038-.042 9" .040-.045
11" .032-.032 11" .038-.040
13" .028-.032 13" .030-.035
15" .030-.035 15" .028-.032
17" .030-.035 17" .027-.032

Barrels again are 28" long, and have not been cut.

I agree with Jim that a full length taper would most likely act as little to no constriction. I've yet to find a full length taper barrel for testing. I do have a 7" taper full choke that acts as a full. As above, I do believe that the diameter immediately in front of the constriction is the controlling diameter.