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 Post subject: Bore diameter - 18.4 vs. 18.7 - Difference
PostPosted: Thu Feb 20, 2020 10:06 am 
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Joined: Tue Aug 14, 2007 11:44 pm
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Location: Denmark
What is the difference between a shotgun with bore 18.4 vs. 18.7 (I'm not looking for the answer 0.3 :-)). Is there any difference when talking about felt recoil, pellet distribution or?




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 Post subject: Re: Bore diameter - 18.4 vs. 18.7 - Difference
PostPosted: Thu Feb 20, 2020 10:09 am 
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I can't tell a difference. But that was before I knew there are differences in bore diameter. Now that I know there is a difference, I still can't tell.

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 Post subject: Re: Bore diameter - 18.4 vs. 18.7 - Difference
PostPosted: Thu Feb 20, 2020 10:19 am 
Field Grade

Joined: Thu Jul 21, 2011 8:03 pm
Posts: 40
Read Sporting Shotgun Performance. On Amazon. Answer - backboard barrels do little and, if the vendor keeps wall thickness the same for proofing but increases ID, you have more metal and often the balance of the gun is off. In your case, the difference is so small, I doubt it means anything


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 Post subject: Re: Bore diameter - 18.4 vs. 18.7 - Difference
PostPosted: Tue Apr 14, 2020 9:51 pm 
Crown Grade

Joined: Fri Mar 23, 2012 9:26 pm
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Location: Southbury, Connecticut
My apologies for delay, but here is my take on 18.4 vs. 18.7
18.4 = .724"
18.7 = .736
Obviously a big difference.
Let me explain why.
Basically bore diameter is critically important when it comes to shotgunning. Way more than most understand or speak of.
Basically any 12 gauge tighter/smaller than .725" (seven hundred twenty five thousandths of an inch) is considerably tight shooting with no choke tube installed at all. In other words, if you shoot a Benelli, older mobil choked Beretta, Remington Rem Choke or perazzi 18.4, then your patterns are starting out like most other guns with a light modified choke in the gun.
Let me give a quick example:
Basically a modified constriction is defined as .020" (twenty thousandths of an inch) constriction from the bore diameter.
If we take a Beretta 303, 390 or urika 391 or Benelli M1 for example....most of these barrels bores measure .719 - .722. Lets use .720 for a round number.
Now, let's take a Browning Invector Plus choked gun such as a 425, 525, 625, GTI, XS, etc.
These guns come with a bore diameter of .740 - .745. We will use .740 for a round number.
As you can easily see with simple math....these guns have a bore diameter difference of .020" (yes...thats a difference of a modified choke amount of constriction).
SO........If you were to take a so called Skeet Choke which is by definition .005" (five thousandths of an inch) constriction, you can see the Berettas and Benelli would have an EXIT diameter of .715" and the brownings would have an exit diameter of .735"
So...with that as a known, let me tell you through thousands of rounds of pattern testing, I assure you, the .715" Mobil choked guns will produce a defined Modified to Improved modified choke pattern at 40 yards with a Skeet choke.
However, the .735" Brownings will produce a Cylinder/Skeet pattern at 40 yards.
Why is this important other than the obvious??
Well....if you are shooting skeet, then you would want an 18.7 bore.
If you are shooting Trap, you would have better ballistics starting with a 18.4
I hope this clarifies a little of the science/physics behind shotgunning, bore diameters and patterns.
Jim Muller
If you want to know a lot more in regards to all of this and much more, watch for my shotgunning book coming out soon.


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 Post subject: Re: Bore diameter - 18.4 vs. 18.7 - Difference
PostPosted: Wed Apr 15, 2020 1:23 am 
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JesperGreth wrote:
What is the difference between a shotgun with bore 18.4 vs. 18.7 (I'm not looking for the answer 0.3 :-)). Is there any difference when talking about felt recoil, pellet distribution or?


See: http://www.claytargettesting.com/Bore_D ... ameter.pdf

The question posed was straightforward, “Have Today's Increased Bore Sizes Improved Trap Gun Performance?” The best available version of the truth is, no, they have done nothing. Neil Winston finishes his study, commenting in part:

“ . . . it is reasonable to conclude that good guns are insensitive to their bore diameters in the range of 0.724" to 0.745" and the near-universal move by manufacturers from the classic dimensions to the modern ones has not improved the patterns produced by their guns at all.

This result is consistent with Dr. Jones’ conclusion on page 153 of Sporting Shotgun Performance: “The story of over sized (Back-Bored in this case) barrels is no story.”

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 Post subject: Re: Bore diameter - 18.4 vs. 18.7 - Difference
PostPosted: Wed Apr 15, 2020 5:34 pm 
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Joined: Sun Jan 17, 2016 3:34 pm
Posts: 5857
RandyWakeman wrote:
JesperGreth wrote:
What is the difference between a shotgun with bore 18.4 vs. 18.7 (I'm not looking for the answer 0.3 :-)). Is there any difference when talking about felt recoil, pellet distribution or?


See: http://www.claytargettesting.com/Bore_D ... ameter.pdf

The question posed was straightforward, “Have Today's Increased Bore Sizes Improved Trap Gun Performance?” The best available version of the truth is, no, they have done nothing. Neil Winston finishes his study, commenting in part:

“ . . . it is reasonable to conclude that good guns are insensitive to their bore diameters in the range of 0.724" to 0.745" and the near-universal move by manufacturers from the classic dimensions to the modern ones has not improved the patterns produced by their guns at all.

This result is consistent with Dr. Jones’ conclusion on page 153 of Sporting Shotgun Performance: “The story of over sized (Back-Bored in this case) barrels is no story.”



Ever considered purchasing your own forum?.........oh, wait.


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 Post subject: Re: Bore diameter - 18.4 vs. 18.7 - Difference
PostPosted: Wed Apr 15, 2020 5:54 pm 
Tournament Grade

Joined: Sun Dec 01, 2019 10:17 pm
Posts: 268
Location: Trinidad, CO
Jimmy Muller wrote:
My apologies for delay, but here is my take on 18.4 vs. 18.7
18.4 = .724"
18.7 = .736
Obviously a big difference.
Let me explain why.
Basically bore diameter is critically important when it comes to shotgunning. Way more than most understand or speak of.
Basically any 12 gauge tighter/smaller than .725" (seven hundred twenty five thousandths of an inch) is considerably tight shooting with no choke tube installed at all. In other words, if you shoot a Benelli, older mobil choked Beretta, Remington Rem Choke or perazzi 18.4, then your patterns are starting out like most other guns with a light modified choke in the gun.
Let me give a quick example:
Basically a modified constriction is defined as .020" (twenty thousandths of an inch) constriction from the bore diameter.
If we take a Beretta 303, 390 or urika 391 or Benelli M1 for example....most of these barrels bores measure .719 - .722. Lets use .720 for a round number.
Now, let's take a Browning Invector Plus choked gun such as a 425, 525, 625, GTI, XS, etc.
These guns come with a bore diameter of .740 - .745. We will use .740 for a round number.
As you can easily see with simple math....these guns have a bore diameter difference of .020" (yes...thats a difference of a modified choke amount of constriction).
SO........If you were to take a so called Skeet Choke which is by definition .005" (five thousandths of an inch) constriction, you can see the Berettas and Benelli would have an EXIT diameter of .715" and the brownings would have an exit diameter of .735"
So...with that as a known, let me tell you through thousands of rounds of pattern testing, I assure you, the .715" Mobil choked guns will produce a defined Modified to Improved modified choke pattern at 40 yards with a Skeet choke.
However, the .735" Brownings will produce a Cylinder/Skeet pattern at 40 yards.
Why is this important other than the obvious??
Well....if you are shooting skeet, then you would want an 18.7 bore.
If you are shooting Trap, you would have better ballistics starting with a 18.4
I hope this clarifies a little of the science/physics behind shotgunning, bore diameters and patterns.
Jim Muller
If you want to know a lot more in regards to all of this and much more, watch for my shotgunning book coming out soon.


Thank you! I had not trouble grasping what you were explaining.

s/f Steve


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 Post subject: Re: Bore diameter - 18.4 vs. 18.7 - Difference
PostPosted: Sun Apr 19, 2020 1:28 pm 
Crown Grade

Joined: Fri Mar 23, 2012 9:26 pm
Posts: 2811
Location: Southbury, Connecticut
RandyWakeman wrote:
JesperGreth wrote:
What is the difference between a shotgun with bore 18.4 vs. 18.7 (I'm not looking for the answer 0.3 :-)). Is there any difference when talking about felt recoil, pellet distribution or?


See: http://www.claytargettesting.com/Bore_D ... ameter.pdf

The question posed was straightforward, “Have Today's Increased Bore Sizes Improved Trap Gun Performance?” The best available version of the truth is, no, they have done nothing. Neil Winston finishes his study, commenting in part:

“ . . . it is reasonable to conclude that good guns are insensitive to their bore diameters in the range of 0.724" to 0.745" and the near-universal move by manufacturers from the classic dimensions to the modern ones has not improved the patterns produced by their guns at all.

This result is consistent with Dr. Jones’ conclusion on page 153 of Sporting Shotgun Performance: “The story of over sized (Back-Bored in this case) barrels is no story.”



Hi Randy, I am not sure what you are agreeing with. But as I explained in detail above in regards to bore diameters and the differences they translate to in patterning is proven.
Maybe Neil Winston's "study" did not take into consideration that the chokes being used in the different bore diameters were incorrect geometries for the bore diameters they were being used for, in turn shooting high deviation patterns (which is what I found) and maybe the patterns were so horribly inconsistent that he did not see a considerable difference in bore diameter efficiency (which is also what I found).
This is one of the reasons I made my own chokes back in 1993. As I stated above in great detail.....Bore diameters pattern differently and the geometries in the choke tube used in each and every bore diameter must have specific geometries for THAT bore diameter in order to offer the utmost pattern efficiency. If it does not, then you have no idea what you are shooting for percentages and thousandths of an inch constriction is grossly secondary to geometry.


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 Post subject: Re: Bore diameter - 18.4 vs. 18.7 - Difference
PostPosted: Sun Oct 11, 2020 4:38 am 
Field Grade

Joined: Fri Aug 09, 2013 7:06 am
Posts: 94
Jimmy Muller wrote:
My apologies for delay, but here is my take on 18.4 vs. 18.7
18.4 = .724"
18.7 = .736
Obviously a big difference.
Let me explain why.
Basically bore diameter is critically important when it comes to shotgunning. Way more than most understand or speak of.
Basically any 12 gauge tighter/smaller than .725" (seven hundred twenty five thousandths of an inch) is considerably tight shooting with no choke tube installed at all. In other words, if you shoot a Benelli, older mobil choked Beretta, Remington Rem Choke or perazzi 18.4, then your patterns are starting out like most other guns with a light modified choke in the gun.
Let me give a quick example:
Basically a modified constriction is defined as .020" (twenty thousandths of an inch) constriction from the bore diameter.
If we take a Beretta 303, 390 or urika 391 or Benelli M1 for example....most of these barrels bores measure .719 - .722. Lets use .720 for a round number.
Now, let's take a Browning Invector Plus choked gun such as a 425, 525, 625, GTI, XS, etc.
These guns come with a bore diameter of .740 - .745. We will use .740 for a round number.
As you can easily see with simple math....these guns have a bore diameter difference of .020" (yes...thats a difference of a modified choke amount of constriction).
SO........If you were to take a so called Skeet Choke which is by definition .005" (five thousandths of an inch) constriction, you can see the Berettas and Benelli would have an EXIT diameter of .715" and the brownings would have an exit diameter of .735"
So...with that as a known, let me tell you through thousands of rounds of pattern testing, I assure you, the .715" Mobil choked guns will produce a defined Modified to Improved modified choke pattern at 40 yards with a Skeet choke.
However, the .735" Brownings will produce a Cylinder/Skeet pattern at 40 yards.
Why is this important other than the obvious??
Well....if you are shooting skeet, then you would want an 18.7 bore.
If you are shooting Trap, you would have better ballistics starting with a 18.4
I hope this clarifies a little of the science/physics behind shotgunning, bore diameters and patterns.
Jim Muller
If you want to know a lot more in regards to all of this and much more, watch for my shotgunning book coming out soon.


So if a Mobil choked barreled gun shoots like a Mod to IMod with just the bore or a Cylinder choke, this would suggest no constriction/choke in a Mobil choked barrel would perform like a U2 or U3, yet you market a U0 and U1 for Mobil Choked guns which, according to you, a U0/U1 like pattern can not be achieved in a Mobil barreled gun due to the bore.

For what it is worth, I have never seen a Mobil Choked gun with a sloppy choke (Cylinder, Skeet, or even IC) hit targets as hard as my .732 bore gun with a .018 choke labeled M.

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Last edited by Hwcn on Mon Oct 12, 2020 7:09 am, edited 1 time in total.

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 Post subject: Re: Bore diameter - 18.4 vs. 18.7 - Difference
PostPosted: Sun Oct 11, 2020 6:57 am 
Presentation Grade

Joined: Fri Jul 24, 2020 2:22 pm
Posts: 687
This brings back the focus on a question I asked a little while ago. Is it the final bore diameter at the exit end of the choke or the amount of constriction from the barrel bore that makes the difference how a gun patterns? Would a straight bore unchoked 0.707” barrel shoot the same pattern as a .732” barrel with a 0.025” constriction choke? Or would .707” barrel and .732” barrels shoot the same pattern when equipped with the same amount of constriction, .025” yielding .682” and .707” respectively? Most people here keep saying it is only constriction that matters, i.e. the change in orifice at the muzzle determines the spread of the shot. But Jimmy is saying the opposite, that it is the final orifice only that determines the pattern, not whether the bore changes due to a constriction at the muzzle end. One would think our experience with various gauge guns would inform our understanding of this question.

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 Post subject: Re: Bore diameter - 18.4 vs. 18.7 - Difference
PostPosted: Sun Oct 11, 2020 10:32 am 
Crown Grade

Joined: Sat Mar 12, 2016 10:39 pm
Posts: 4940
It's the amount of constriction from the barrel bore diameter that matters.

Otherwise, "jug choking" a method used to increase a barrel's choke by increasing the bore diameter of the barrel, wouldn't work.

*I'm not sure if "jug choking" is the correct term.

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-Political Correctness
-Insurance
-Securitization
Take your pick.

Always make an even number of mistakes. One may cancel out another.

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 Post subject: Re: Bore diameter - 18.4 vs. 18.7 - Difference
PostPosted: Sun Oct 11, 2020 10:36 am 
Presentation Grade

Joined: Fri Jul 24, 2020 2:22 pm
Posts: 687
Zbigniew wrote:
It's the amount of constriction from the barrel bore diameter that matters.

Otherwise, "jug choking" a method used to increase a barrel's choke by increasing the bore diameter of the barrel, wouldn't work.

*I'm not sure if "jug choking" is the correct term.

Good point, but your position directly co read its Jimmy Miller’s. He is supposed to be an authority. Something’s gotta give.

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 Post subject: Re: Bore diameter - 18.4 vs. 18.7 - Difference
PostPosted: Tue Nov 10, 2020 1:15 pm 
Crown Grade
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Joined: Tue Mar 06, 2007 5:21 pm
Posts: 5197
Location: UK
Jimmy Muller wrote:
My apologies for delay, but here is my take on 18.4 vs. 18.7
18.4 = .724"
18.7 = .736
Obviously a big difference.
Let me explain why.
Basically bore diameter is critically important when it comes to shotgunning. Way more than most understand or speak of.
Basically any 12 gauge tighter/smaller than .725" (seven hundred twenty five thousandths of an inch) is considerably tight shooting with no choke tube installed at all. In other words, if you shoot a Benelli, older mobil choked Beretta, Remington Rem Choke or perazzi 18.4, then your patterns are starting out like most other guns with a light modified choke in the gun.
Let me give a quick example:
Basically a modified constriction is defined as .020" (twenty thousandths of an inch) constriction from the bore diameter.
If we take a Beretta 303, 390 or urika 391 or Benelli M1 for example....most of these barrels bores measure .719 - .722. Lets use .720 for a round number.
Now, let's take a Browning Invector Plus choked gun such as a 425, 525, 625, GTI, XS, etc.
These guns come with a bore diameter of .740 - .745. We will use .740 for a round number.
As you can easily see with simple math....these guns have a bore diameter difference of .020" (yes...thats a difference of a modified choke amount of constriction).
SO........If you were to take a so called Skeet Choke which is by definition .005" (five thousandths of an inch) constriction, you can see the Berettas and Benelli would have an EXIT diameter of .715" and the brownings would have an exit diameter of .735"
So...with that as a known, let me tell you through thousands of rounds of pattern testing, I assure you, the .715" Mobil choked guns will produce a defined Modified to Improved modified choke pattern at 40 yards with a Skeet choke.
However, the .735" Brownings will produce a Cylinder/Skeet pattern at 40 yards.
Why is this important other than the obvious??
Well....if you are shooting skeet, then you would want an 18.7 bore.
If you are shooting Trap, you would have better ballistics starting with a 18.4
I hope this clarifies a little of the science/physics behind shotgunning, bore diameters and patterns.
Jim Muller
If you want to know a lot more in regards to all of this and much more, watch for my shotgunning book coming out soon.
Errm.. Sorry, but this is all completely wrong. Pattern size is determined by the difference between the exit diameter and the bore diameter. A few thou either way in the bore size has no bearing whatsoever on how the system works. It's the choke constriction relative to the bore size that matters and it applies equally for a .724 bore or a .740 bore.



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