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Lawyer and Gun Activist
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Has anyone here had a Gunsmith lengthen a shotgun chamber from 2.75" to 3 inch, or in a related note, polish out the forcing cone area to make it more gradual, more tapered?

Who in the Northern Metro Atlanta area can do this and does anybody have an idea what it would cost?


I have an early 1970's vintage marlin model 55 but it is not the "goose gun" version (goose guns had a 3 inch chamber), this has a 2.75 inch chamber. The gun seems to be only suitable for turkey or migratory waterfowl hunting due to the long barrel, full choke, and bolt action mechanism. But waterfowl hunting is usually done with 3 inch ammunition, and it is almost impossible to find suitable ammo loaded in 2.75" chamber shells. Everybody sells 3 inch shells with that kind of shot payload.
 

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Lawyer and Gun Activist
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Discussion Starter #4
$42 bucks seems worth it.
I was thinking about spending $20 to get a brake cylinder honing tool which has three flat abrasive stones that gets spun on the end of a drill and will pull out away from the center of the bore / cylinder with centripetal force ...

...but in the folded position they almost fill up the available space, so they'd hardly have a chance to expand in a 12 gauge chamber. So they won't be pushing up against the walls with much force, and I think it would take a long time to polish out that much barrel steel.
 
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$42 bucks seems worth it.
I was thinking about spending $20 to get a brake cylinder honing tool which has three flat abrasive stones that gets spun on the end of a drill and will pull out away from the center of the bore / cylinder with centripetal force ...

...but in the folded position they almost fill up the available space, so they'd hardly have a chance to expand in a 12 gauge chamber. So they won't be pushing up against the walls with much force, and I think it would take a long time to polish out that much barrel steel.
I doubt it would work. That is simply honing and it would take forever and a half to remove any serious amount of material.
 

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I have an old brake hone of you want to borrow it and try. I agree it likely won't remove enough material quickly enough to matter. You might want to use it to smooth out the chamber after the reamer. Remember it uses brake fluid as the lubricant.
 

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Discussion Starter #8
Well, I have a corded electric drill, so I can do a bunch of spinning.
 

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Man of Myth and Legend
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How are you going to get the length proper if you have a hone? Seems like it might be a bit risky to cut it too long. Don't want to get into the area where it starts to neck down from chamber to just barrel, leaving some critical part a bit too thin.

If it happens be sure to post pics of the newly created 2 7/8" barrel. Spend the $42 and get the proper tool.

Nemo
 

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My lathe only has a 3/4” spindle bore otherwise I’d offer that you could drop by with go/no-go gauges and nibble away at it with a boring bar.
 

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Will the recut barrel contain the blast of a three inch shell for thousands of rounds?
 

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Man of Myth and Legend
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Whatever method he uses I still want to see the pics of that shortened barrel. And any other shortened appendages that he gets at the using it.

Nemo
 

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Discussion Starter #14
SAAMI specs for chamber pressure has the same maximum listed for 2.75" inch 12 gauge and 3-inch
twelve gauge. It's not until you get up to 3 1/2 inch shells that the pressure limits are a bit higher, and even those readings are still only 1/4 of what a centerfire hunting rifle generates.

3" shells very expensive, and if I modified my gun to accept them, I don't think any future user of this gun would fire them more than a box of them per year.

I should probably try to avoid lengthening the chamber too much so that 3 1/2 inch magnum shells will fit in there. If somebody touched off one of those it might be a problem, although I think the most realistic danger is having the wooden stock crack rather than the barrel explode.
 

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Man of Myth and Legend
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I am not concerned about chamber pressure. I was thinking of weakened chambers handling the pressure.

Either way, post the pics. Things go real bad you will need to change from gunsmoker to gunstump.

Nemo
 

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Discussion Starter #16
Why would a chamber that's 0.25" longer be "weaker" than one of the standard length, if shells with identical chamber pressure are fired in both?

The only answer I can think of would only exist where a gun's barrel is suddenly and dramatically narrowed and thinned just forward of the shorter chamber.

Shotguns are generally not contoured that way, though some light-sporter big game rifle barrels have such a contour.
 

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All of mine have had a narrowing.

Nemo
 

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Why would a chamber that's 0.25" longer be "weaker" than one of the standard length, if shells with identical chamber pressure are fired in both?

The only answer I can think of would only exist where a gun's barrel is suddenly and dramatically narrowed and thinned just forward of the shorter chamber.

Shotguns are generally not contoured that way, though some light-sporter big game rifle barrels have such a contour.
I have an inexpensive double barrel where the chambers are clearly machined cylinders welded onto tapered barrels. If they weren't already 3", I certainly wouldn't look at reaming those, but that's a different firearm than the OP.

To do some more research on a Marlin 55, does it have a serial number? Can you post anything about the markings? I ask because if it was a 70s vintage goose gun it may already be 3" gun just not marked as one. YMMV, go slow, use caution.
 

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Why would a chamber that's 0.25" longer be "weaker" than one of the standard length, if shells with identical chamber pressure are fired in both?
GS, a 3D CAD drawing of the OD and ID as it exists now will give you the chamber wall thickness. If the OD taper starts near the end of the 2.75" chamber then it may be a No/Go.
 
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