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[Backboring] [Chokes] [Forcing Cones] [Patterns] [Shot Size Count] [Barrel Length Velocity's]

 

 

 

 

 

Backboring

 

Backboring is a term that is mostly misunderstood as being the forcing cone due to the fact that when working on the forcing cone area you are working on the back end of the barrel then afterwards the barrel is supposedly ''backbored''  but in reality it is the actual bore of the gun from just ahead of the forcing cone all the way through the barrel to the choke.

When you backbore a barrel you are enlarging the bore size of the barrel. When this is done it does several things: It usually is done to increase the amount of choke with a given choke diameter.

A back-bored barrel is a shotgun barrel that has a bore diameter that is increased beyond the minimum specifications. Increasing the bore diameter of a shotgun barrel greatly enhances its performance. As a larger bore diameter reduces friction of the shot charge against the barrel wall. Instead of trying to overcome friction, powder gases expend more energy on the wad base, resulting in an increase in shot velocity. Because there is less constriction or pressure from the barrel walls on a shot charge, there are fewer deformed shot pellets. A greater number of ballistically superior round pellets in a shot charge gives you exceptionally uniform patterns -- delivering more shot pellets in the effective part of your pattern.

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Example :

The standard bore size for a 12 gauge is .729 thousandths and with a full choke tube size of .690  you will have .039 thousandths of choke.

When the bore size is enlarged even more , say to .742 thousandths then you would have .052 thousandths of choke with the same .690 choke tube.


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Backboring can improve a shotgun's capabilities to a certain extent but has its exceptions. Backboring also is to be believed to increase velocity to some extent and also it shortens the shot columns length during flight and this will enable more shot to hit the target at one time which is better for making clean kills on game.

One of the exceptions to backboring is that if you get too big with the bore size for a given gauge size then you are going to have problems with hot gasses blowing by the wadding upon firing. This is going to interfere with the shot charge severely upon exiting the barrel and you will get poor results from your patterns.

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Example of too large a  boresize :

Take a 12 gauge size wad which is designed to seal the gas pressures behind the wad and fire it from a 10 gauge bore size ,  you will have a severe gas blowby condition which will result in blown patterns.

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When you backbore a barrel, you are limited as to how big you can get when you are using a choke tube in the barrel. It is critical that the bore doesn't get larger than the  rear skirt area of the choke tube. If this happens, then the choke will be blown out and damage the barrel.

As you can see, there is much more to backboring than just boring the barrel out to enlarge the bore size without considering the results. This is best left to someone who understands the aspects of it.

 Below is a diagram of the inside diameter of standard bore sizes of 6 common shotgun gauges

Minimum and Maximum Bore Diameters

 

Standard 10 10 12 12 16 16 20 20 28 28 410 410
Maximum and Minimum Inches Min In. Max In. Min In. Max In. Min In. Max In. Min In. Max In. Min In. Max In. Min In. Max In.
US................. (ANS Z299.2) .775 .795 .725 .745 .665 .685 .615 .635 .545 .565 .410 .430
England (1954) .751 .802 .710 .750 .637 .676 .596 .636 .543 .562 .405 .416
Germany (1939) .759 .775 .716 .732 .661 .677 .618 .633 .543 .559 .401 .417
Japan (JISB 9804) .759 .775 .712 .728 .661 .677 .614 .629 .551 .566 .409 .425

 

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Chokes

Chokes work by constricting the shot charge by a given amount , which is usually measured in thousandths of an inch.

12 gauge

20 gauge

American

English

Browning

Italian

Percentage

Constriction

Constriction

Designation

Designation

Codes

Codes

Pattern

.000 - .001

.000 - .001

cylinder

true cylinder

***

C***

40%

.002 - .006

.002 - .004

skeet

imp.cylinder

**$

S

45%

.007 - .013

.005 - .011

imp.cylinder

1/4

**-

****

55%

.014 - .023

.012 - .019

modified

1/2

**

***

60%

.024 - .031

.020 - .026

imp.modified

3/4

*-

**

65%

.032 - .040

.027 - .033

full

full

*

*

70%

.040 + 

.034 +

extra full

 

 

 

75%

The constriction of a choke is determined by the actual difference between the tightest inside diameter measurement of the choke , and the actual bore diameter of the barrel.

Measuring your choke dimension only goes so far. Just because a choke measures a certain dimension doesn't mean that it will shoot what it should because different guns with the same choke constriction will shoot different patterns. You have to test shoot it if you really want to know what it's doing , regardless of what your choke is marked. Choke alone does not determine whether a particular gun will shoot to its fullest potential.

There are so many variables to the designs of chokes such as ; short verses long , conical verses conical parallel , wad stop designs verses slick chokes , lead shot designs verses steel shot designs.

The list goes on and on as to what choke is best for a particular gun and what its purpose is. To get the best performance from your gun, you may need to experiment with various choke sizes and loads to get the top performance from your gun.

Tight , uniform patterns require that everything be carefully controlled in the shotgun and ammunition being used.

 

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Forcing Cones

 

This is the area just ahead of the chamber that is designed to introduce  the shot charge into the actual bore of the barrel 

This transition has alot to do with how the gun performs with given loads. Most factory guns have a forcing cone length of only 1/4 inch to 1/2 inch long and this short abrupt taper into the barrel deforms a lot of shot upon firing a given load.

By lengthening the forcing cone out to around one and one half inches or longer , this reduces the angle of the taper leading into the barrel and reduces the deformation of the shot charge.

You can improve patterns by as much as 15 percent and also the long forcing cone will reduce recoil because the shot charge has less resistance going into the barrel hence the less felt recoil.

By extending this tapered section you do not actually reduce the amount of recoil, but rather extend the period of time that the recoil is felt. Therefore, it feels like a reduction of recoil.(lower pressure peak)Because the transition is longer, fewer shot become deformed as it travels through it. The less deformed shot there is then the more dense the pattern becomes and this is how the patterns improve

Below is a picture showing an actual cutaway view of the forcing cone area that's inside most all shotgun barrels , notice the upper barrel is showing a lengthened forcing cone while the lower barrel is shown with a factory length forcing cone

 

 

 

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Patterns

 

All barrels and chokes are not constricted equally despite their markings.

Pattern performance depends on other factors besides just choke. The only way to really know what a given gun and choke combination is doing is to actually pattern it.

The best way is to go to your local hardware or builders supply store and get a roll of red rosin paper which is 36 inches in width, then cut you a piece off to use as a target and place it at 40 yards, then shoot this target with the gun and choke combination with the shot size that you plan on using.

After shooting the target draw a 30 inch circle around the most dense part of the load and count all the pellets within the 30 inch circle. Then you can figure the percentage of your gun by dividing the number of pellets that are within the circle by the number of total shot that was in the load that was fired.

Example: If you were shooting a 2 oz. Load of  #4 shot which has 270 pellets in the load then after firing and counting the pellets within the 30 inch circle that you had 250 shot inside , then 250 divided by 270 equals a 92.5 percent pattern.

This is the only way to know what your gun is really doing.

 

Diameter off pattern spread
(covered by the main body of the charge at listed ranges)

Choke

20 Yards

30 Yards

40 Yards

Cylinder

32 inches

44 inches

58 inches

Imp. Cyl.

26 inches

38 inches

51 inches

Modified

21 inches

32 inches

45 inches

Full

16 inches

27 inches

40inches

 

Pattern Percentages Per Choke Per Yard
(using 30 inch test target)

Yards

10

15

20

25

30

35

40

45

50

55

60

Full

-

-

-

100 90 80 70 60 50 40

30

Modified

-

-

100

90

80

70

60

50

40

30

20

Imp.Cyl

-

100

90

80

70

60

50

40

30

20

10

Cylinder

100

90

80

70

60

50

40

30

20

10

-

 

Total Spread of Pattern Diameter Per Choke and Per Yard
(using 50 Inches as Maximum Spread)  

Yards

10

15

20

25

30

35

40

45

50

Full

10''

15''

20''

25''

30''

35''

40''

45''

50''

Modified

15''

20''

25''

30''

35''

40''

45''

50''

-

Imp.Cyl

20''

25''

30''

35''

40''

45''

50''

-

-

Cylinder

25''

30''

35''

40''

45''

50''

-

-

-

 

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Standard Shot Chart

This tabulation gives the approximate number of pellets per shotshell load for shot sizes 2 through 9. The exact number of pellets will vary, depending on exact alloy content: for example, chilled shot vs. soft shot. Variations in shot pellet diameter will also affect the exact number of pellets per load, when shot charge is thrown volumetrically (rather than weight).

Shot Charge

#2

#4

#5

#6

#7

#7-1/2

#8

#8-1/2

#9

Shot Charge

1/2 oz.

45

67

85

112

150

175

205

242

292

1/2 oz.

5/8 oz.

55

85

108

139

187

216

256

300

366

5/8 oz.

3/4 oz.

67

101

127

168

224

262

308

363

439

3/4 oz.

7/8 oz.

79

118

149

197

262

306

359

425

512

7/8 oz.

1 oz.

90

135

170

225

299

350

410

485

585

1 oz.

1-1/8 oz.

101

152

191

253

336

393

461

545

658

1-1/8 oz.

1-1/4 oz.

112

169

213

281

374

437

513

605

731

1-1/4 oz.

1-3/8 oz.

124

186

234

309

411

481

564

665

804

1-3/8 oz.

1-1/2 oz.

135

202

255

337

449

525

615

730

877

1-1/2 oz.

1-5/8 oz.

146

220

276

366

486

569

666

790

951

1-5/8 oz.

1-7/8 oz.

169

253

319

422

561

656

769

850

1097

1-7/8 oz.

2 oz.

180

270

340

450

598

700

820

970

1170

2 oz.

2-1/8 oz.

187

289

366

474

635

733

869

1020

1243

2-1/8 oz.

2-1/4 oz.

202

304

382

506

672

786

922

1090

1316

2-1/4 oz.

 

Component Shot Chart - Diameter in Inches

Winchester uniform, chilled lead shot provides consistent shot patterns and better penetration. Strict quality control throughout the manufacturing process assures the ultimate in performance.

12*

11*

10*

9*

8*

7-1/2*

7*

6*

5*

4*

2*

BB*

No. 4 Buck** No. 00**

.050

.060

.070

.080 .090

.095

.100

.110

.120

.130

.150

.180

.240

.330

* - Approximate number of pellets to the ounce
** - Approximate number of pellets to the pound

#12

2835

#11

1380

#10

868

#9

600

#8

409

#71/2

345

#7

299

#6

223

#5

170

#4

135

#2

90

BB

58

No. 4 Buck**

344

No. 00**

115

 

Loads per Pound

1/2 oz.

32

5/8 oz.

26

3/4 oz.

21

7/8 oz.

18

1 - oz.

16

1-1/8 oz.

14

1-1/4 oz.

13

1-3/8 oz.

12

1-1/2 oz.

11

1-5/8 oz.

10

1-3/4 oz.

9

1-7/8 oz.

9

2 -oz.

8

2-1/8 oz.

8

 

 

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Shotgun Velocity Variations with Different Barrel Lengths

* Denotes Barrel with .660 Turkey Choke Installed

* * Denotes Manufacturer's Published Velocity , Typically Fired from a 30 inch Test Barrel

Manufacturer

Load

Barrel
Length

Avg.Velocity
fps@ 3 Feet

Winchester Western

1 3/4 oz. No. 5

22

1,228

Winchester Western

1 3/4 oz. No. 5

24

1,227

Winchester Western

1 3/4 oz. No. 5

26

1.266

Winchester Western

1 3/4 oz. No. 5

28

1,292

Winchester Western

1 3/4 oz. No. 5

28*

1,352

Winchester Western

1 3/4 oz. No. 5

30**

1,300

Winchester Western

2 oz. No. 5

22

1.124

Winchester Western

2 oz. No. 5

24

1.124

Winchester Western

2 oz. No. 5

26

1,129

Winchester Western

2 oz. No. 5

28

1,147

Winchester Western

2 oz. No. 5

28*

1,220

Winchester Western

2 oz. No. 5

30**

1,125

Federal

1 3/4 oz. No. 4

22

1,251

Federal

1 3/4 oz. No. 4

24

1,240

Federal

1 3/4 oz. No. 4

26

1,269

Federal

1 3/4 oz. No. 4

28

1,288

Federal

1 3/4 oz. No. 4

28*

1,340

Federal

1 3/4 oz. No. 4

30**

1,300

Federal

2 oz. No. 4

22

1,069

Federal

2 oz. No. 4

24

1,094

Federal

2 oz. No. 4

26

1,103

Federal

2 oz. No. 4

28

1,124

Federal

2 oz. No. 4

28*

1,173

Federal

2 oz. No. 4

30**

1,175

Remington

2 oz. No. 4

22

1,040

Remington

2 oz. No. 4

24

1,068

Remington

2 oz. No. 4

26

1,059

Remington

2 oz. No. 4

28

1,078

Remington

2 oz. No. 4

28*

1,172

Remington

2 oz. No. 4

30**

1,175

Kent

1 5/8 oz. No 5

22

1,213

Kent

1 5/8 oz. No 5

24

1,226

Kent

1 5/8 oz. No 5

26

1,257

Kent

1 5/8 oz. No 5

28

1,273

Kent

1 5/8 oz. No 5

28*

1,366

Kent

1 5/8 oz. No 5

30**

1,330

 

 

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