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Old 07-23-2014, 22:37   #1
RWBlue
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Inside diameter of shotgun tube

We all know that a 12ga is a .58 cal bore, but it isn't exactly.

Can someone point me to a link that shows the inside diameters of a 12ga?

I am looking for diameters..
There is a cylinder bore. What are the other bores?
There are chokes.

Or to put it in a more practical way..
I have a MP94 on the way.
I have an 870.
I have an old, change that, very old single barrel.
I have access to a Sears and Robucks JC Higgings with full choke.
If I bought an expensive double how would that be different?
If I bought a coach gun, how would that be different?
How are they different inside the barrel?
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Old 07-23-2014, 23:26   #2
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Quote:
Originally Posted by RWBlue View Post
We all know that a 12ga is a .58 cal bore, but it isn't exactly.

Can someone point me to a link that shows the inside diameters of a 12ga?

I am looking for diameters..
There is a cylinder bore. What are the other bores?
There are chokes.

Or to put it in a more practical way..
I have a MP94 on the way.
I have an 870.
I have an old, change that, very old single barrel.
I have access to a Sears and Robucks JC Higgings with full choke.
If I bought an expensive double how would that be different?
If I bought a coach gun, how would that be different?
How are they different inside the barrel?
12 Gauge is ~.729", not .58". SAAMI specs are .725"-.745", but most are .729" from what I've seen. Some shotguns are overbored by a few thousandths, others are underbored by a few thousandths.

When people say cylinder "bore", they mean it has no constriction at the end of the barrel (which is what a choke is). All of the actual bores are about the same on standard 12 gauge shotgun barrels, but many have fixed or removable chokes, that constrict the diameter at the end of the barrel.

Look HERE for various choke diameters, though the actual choke name (Cylinder, Improved Cylinder, Modified, etc.) is deemed by the patterns, not the constriction. You can find that info, and a lot more, somewhere HERE (in the sticky at the top of the forum).

When you buy an expensive double, you're paying for fit, finish, regulation, quality of parts, labor in manufacturing, etc. Don't know if they have tighter or looser tolerances in the chamber/bore, I don't own any expensive doubles... But they all come with removable chokes for you to select exactly what you need, and they'll all adhere to SAAMI specs.
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Old 07-24-2014, 09:03   #3
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That is what I get for posting when I am tired, I put in 58.....ugg

What about right in front of the shell?
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Old 07-24-2014, 11:37   #4
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That is what I get for posting when I am tired, I put in 58.....ugg

What about right in front of the shell?
Its more open in front of the shell but it begins to constrict as the barrel has a forcing cone that squeezes the shot cup in a little. There is no set amount and cheaper guns tend to have shorter more abrupt forcing cones while more expensive guns have more gradual forcing cones which helps reduce felt recoil and generally makes for a more consistent pattern.

But what Inebriated posted above is correct.

Keep in mind that a shotgun bore diameter will vary from manufacturer to manufacturer and even among cheaper guns from the same maker. SAAMI tolerances allow for it.
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Old 07-24-2014, 15:22   #5
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Its more open in front of the shell but it begins to constrict as the barrel has a forcing cone that squeezes the shot cup in a little. There is no set amount and cheaper guns tend to have shorter more abrupt forcing cones while more expensive guns have more gradual forcing cones which helps reduce felt recoil and generally makes for a more consistent pattern.

But what Inebriated posted above is correct.

Keep in mind that a shotgun bore diameter will vary from manufacturer to manufacturer and even among cheaper guns from the same maker. SAAMI tolerances allow for it.
Approximately how many inch(es) ahead of the cartridge is it opened up?

When we talking about cheaper guns are we talking Remington and Winchester OR really cheap stuff?
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Old 07-24-2014, 15:35   #6
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Approximately how many inch(es) ahead of the cartridge is it opened up?

When we talking about cheaper guns are we talking Remington and Winchester OR really cheap stuff?
HERE is the spec sheet for 2-3/4" 12 gauge. Go to either PDF page 23 or publication page 17. The listed forcing cone length is .4172"

I don't know what exact length forcing cone is on any gun, I don't really care... I think it's kind of a gimmick. It increases barrel volume, which lowers pressure. That lowers velocity, and most folks will tell you that lower velocity shot tends to pattern better. It is also why the felt recoil is lower. To me, if I want lower recoil and a better pattern, I'll use lower velocity loads.

Keep in mind, this is Tactical Shotguns... Not Olympic Skeet/Trap Shooters. Most of us couldn't care less what the specifics of the bore is... If it puts the amount of shot we want where we want it, and it does it without fail, we're happy.
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Last edited by Inebriated; 07-24-2014 at 15:37..
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Old 07-24-2014, 15:44   #7
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Originally Posted by Inebriated View Post
Keep in mind, this is Tactical Shotguns... Not Olympic Skeet/Trap Shooters. Most of us couldn't care less what the specifics of the bore is... If it puts the amount of shot we want where we want it, and it does it without fail, we're happy.
Normally I am the same way. But right now I have something specific in mind.

Thanks guys, I think I have the answers I need.
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Old 07-25-2014, 18:25   #8
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Normally I am the same way. But right now I have something specific in mind.

Thanks guys, I think I have the answers I need.
Glad we could be of service!
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