JP2018123985A - Bullet for firearm - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a bullet for a firearm capable of reducing an impact it receives when it passes through a barrel.SOLUTION: A bullet for a firearm 10 is configured with an outside of a bullet core 11 covered by a bullet jacket 21. In the bullet jacket 21, parts with the maximum outer diameter in the bullet for a firearm 10 are arranged and at least one maximum outer diameter cylindrical part 31 extending in a shaft direction of the bullet for a firearm 10 is arranged. All of the maximum outer diameter cylindrical parts 31 have an annular gap 32 over the entire area in shaft direction of the bullet for a firearm 10 between the bullet core 11 and itself.SELECTED DRAWING: Figure 1

Description

  The present invention relates to a bullet for a small firearm whose outer side of a bullet core is covered with a shell.

  Patent Document 1 discloses a bullet for a small firearm in which an annular protrusion formed on an inner peripheral surface of a maximum outer diameter portion of an armor is in contact with a bullet core.

Japanese Patent No. 5244010 (paragraph [0018], FIG. 1)

  However, in the bullet for small arms of Patent Document 1, the portion of the maximum outer diameter portion of the armor where the annular protrusion is provided cannot be deformed inward, so when the bullet for the small weapon passes through the barrel. There was a problem that the resistance was increased, the pressure inside the gun cavity was increased, and the barrel was damaged.

  This invention is made | formed in view of the said situation, and aims at provision of the bullet for small arms which can make small the resistance received when passing a barrel.

  In order to achieve the above object, the invention according to claim 1 is directed to a bullet for a small firearm whose outer side is covered with an armor, and the armor has a maximum outer diameter in the bullet for the small weapon. At least one maximum outer diameter cylinder portion extending in the axial direction of the small firearm bullet, and all the maximum outer diameter cylinder portions are in the axial direction of the small firearm bullet. An annular gap is provided between the core and the entire center.

  According to a second aspect of the present invention, in the bullet for a small firearm according to the first aspect, the bullet has a bullet straight portion extending linearly in an axial direction of the bullet for the small weapon, and a warhead from the bullet straight portion. And a tapered head that is tapered toward the side and a truncated cone-shaped bullet tail that extends from the straight center portion to the tail side and is reduced in diameter on the tail side. The armor has a straight cylindrical armor straight portion extending in the axial direction of the bullet for the small arms, and a tapered shape extending from the armor straight portion toward the warhead and corresponding to the bullet head. An armored head, and an armored tail extending from the armored straight part to the bullet side and having a truncated cone shape corresponding to the ballistic tail, wherein the armored head is By fitting with the head of the bullet, the armored tail part is fitted with the ballistic tail part, While holding the bullet center, the armored straight part has an annular gap between the armor straight part over the whole axial direction of the bullet for the small arms, and all the armored straight part The maximum outer diameter cylindrical portion is provided.

  According to a third aspect of the present invention, in the bullet for a small firearm according to the first aspect, the bullet has a bullet straight portion extending linearly in the axial direction of the bullet for the small weapon, and a warhead from the bullet straight portion. A tapered head portion that is tapered toward the side and a tapered tail portion that extends from the bullet straight portion to the tail side and has a truncated cone shape reduced in diameter on the tail side. The armor is configured with a straight cylindrical armor straight portion extending in the axial direction of the bullet for the small arms, and a tapered shape extending from the armor straight portion toward the warhead and corresponding to the bullet head The armored head portion, and the armored straight portion that extends from the armored straight portion to the tail side and has a truncated cone shape corresponding to the center of the ballistic tail, , A constricted part that is depressed on the side of the center of the circumference is formed over the entire circumferential direction, The upper part is fitted with the bullet head, the covered tail part is fitted with the bullet tail part, and the constricted part is fitted with the bullet straight part so that the covered part is the bullet The portion of the straight upper portion excluding the constricted portion has an annular gap with the bullet straight portion over the entire axial direction of the bullet for the small firearm, All the said largest outer diameter cylinder parts are provided in the site | part except the said constriction part among instep straight parts.

  According to a fourth aspect of the present invention, there is provided a bullet for a small firearm according to the third aspect, wherein the bullet is formed of a material different from the first bullet component and the first bullet component. A core component is arranged in the axial direction of the bullet for the small firearm.

  According to a fifth aspect of the present invention, in the bullet for a firearm according to any one of the second to fourth aspects, the axial length of the core straight portion is longer than the axial length of the armored straight portion. And the end of the armored straight part side of the armored straight part has an annular gap between the end of the armored straight part and the end of the armored head part on the armored straight part side. An annular gap is provided between the core straight portion.

[Invention of Claim 1]
According to the first aspect of the present invention, all the maximum outer diameter cylindrical portions can be deformed toward the central axis side of the bullet for the small firearm over the entire axial direction of the bullet for the small firearm. This makes it possible to reduce the resistance received when the firearm bullet passes through the gun cavity.

[Inventions of Claims 2 to 4]
In the invention of claim 2, the armored head is fitted to the bullet head and the armored tail part is fitted to the ballistic tail part so that the armor holds the bullet, so that the fixation of the bullet is stable. Can be achieved.

  Further, in the invention of claim 3, in addition to the fit between the armored head and the bullet head and the fit between the armored tail and the ballistic tail, the ballistic is also achieved by fitting the constricted part and the ball straight part. Therefore, it is possible to further stabilize the fixation of the bullet core.

  In particular, as in the invention of claim 4, when the bullet is composed of two components (first and second bullet components), as in the invention of claim 3, It is preferable to hold the bullets at the three locations of the armored head, the armored tail and the constricted part.

[Invention of claim 5]
In the invention of claim 5, when the maximum outer diameter portion cylinder portion includes the end portion on the armored straight portion side in the armored tail portion and the end portion on the armored straight portion side in the armored head portion, The outer diameter cylindrical portion is easily deformed toward the center axis side of the small firearm bullet, and the resistance received when the small firearm bullet passes through the gun cavity can be further reduced.

Partially broken side view of a bullet for small arms according to the first embodiment (A) Sectional view around the armored head, (B) Sectional view around the armored tail Sectional view showing the process of a small weapon bullet passing through the barrel Sectional view showing the process of a small weapon bullet passing through the barrel Sectional view showing the manufacturing process of bullets for small arms Partially cutaway side view of a bullet for a firearm according to a second embodiment Sectional view showing the manufacturing process of bullets for small arms Partially broken side view of a bullet for a small firearm according to a third embodiment Side sectional view of a bullet for firearms according to another embodiment Side sectional view of a bullet for firearms according to another embodiment Side sectional view of a bullet for firearms according to another embodiment Side sectional view of a bullet for firearms according to another embodiment

[First Embodiment]
DESCRIPTION OF EXEMPLARY EMBODIMENTS Hereinafter, a first embodiment of the invention will be described with reference to the drawings. As shown in FIG. 1, a bullet 10 for a small firearm (hereinafter simply referred to as “bullet 10”) of the present embodiment is formed by covering an outer side of a bullet core 11 with an armor 21. The bullet 10A of the bullet 10 is formed in a tapered shape, and the bullet 10B of the bullet 10 is formed in a truncated cone shape whose diameter is reduced as the distance from the bullet 10A increases. The armor 21 is formed in a container shape in which the warhead 10 </ b> A side is closed, while the bullet 10 </ b> B side is open. In the following, the bullet 10A side of the bullet 10 will be referred to as “front side”, and the bullet 10B side will be referred to as “rear side” as appropriate.

  The bullet 11 includes a bullet straight portion 12 extending linearly along the central axis J1 of the bullet 10, a bullet head 13 extending from the bullet straight portion 12 toward the bullet 10A, and a bullet straight portion 12 And a bullet tail portion 14 extending from the bullet to the bullet 10B side. The armor 21 includes a straight cylindrical armored straight portion 22 extending along the central axis J1 of the bullet 10, an armored head 23 extending from the armored straight portion 22 toward the warhead 10A, And a tortoise tail portion 24 extending from the straight portion 22 to the bullet 10B side.

  The bullet head 13 is formed in a tapered shape that is reduced in diameter toward the front side (as it is away from the bullet straight portion 12), and the armored head 23 is formed in a tapered shape corresponding to the bullet head 13. Yes. Further, the bullet tail portion 14 is formed in a truncated cone shape whose diameter is reduced toward the rear side (as it is away from the bullet straight portion 12), and the tortoise tail portion 24 is a truncated cone shape corresponding to the bullet tail portion 14. Is formed. The armored head 23 is engaged with the ballistic head 13, and the armored tail 24 is fitted with the ballistic tail 14, so that the armored 21 holds the ballistic 11.

  Specifically, the front end surface 13F of the bullet head 13 is flat, while the inner surface of the armored head 23 has a pointed tip. The armored head 23 has a front end gap 23 </ b> S between the front end surface 13 </ b> F of the bullet head 13. The armored tail portion 24 includes a fitting cylindrical wall 24A that fits with the bullet tail portion 14 and a retaining wall 24B that is directed to the outer peripheral portion of the rear surface of the bullet tail portion 14 and prevents the bullet 11 from coming off. It is configured.

  As shown in FIG. 1, the armored straight portion 22 constitutes a portion where the outer diameter is maximum in the bullet 10. That is, in the bullet 10 of the present embodiment, the entire armored straight portion 22 constitutes the maximum outer diameter cylindrical portion 31 of the present invention.

  The inner diameter of the armored straight portion 22 is larger than the outer diameter of the bullet 11 (specifically, the ammunition straight portion 12), and the armored straight portion 22 extends over the entire axial direction of the bullet 10, An annular gap 32 is provided between the bullet core 11.

  As shown in FIGS. 2 (A) and 2 (B), the axial length of the core straight portion 12 is longer than the axial length of the armored straight portion 22. As shown in FIG. 2 (A), the rear end portion of the armored head 23 (the end portion on the armored straight portion 22 side) is a bullet 11 (specifically, the ball straight portion 12). An annular gap 33 is provided between them. Further, as shown in FIG. 2 (B), the front end portion of the armored tail portion 24 (the end portion on the armored straight portion 22 side) is between the core 11 (specifically, the core straight portion 12). An annular gap 34 is provided.

  3 and 4 show the bullet 10 as it passes through the barrel 70. In the example of FIGS. 3 and 4, the bullet 10 is fitted into the front end cylinder portion 42 </ b> A of the cartridge case 42 to constitute a part of the ammunition 41. The barrel 70 is formed with a gun cavity 71 communicating with the cartridge case 42 from the front side. Specifically, the gun cavity 71 includes a guide portion 72 that communicates with the cartridge case 42 and is reduced in diameter toward the front side, and a straight portion 73 that communicates with the guide portion 72 from the front side. The inner diameter of the straight portion 73 is slightly smaller than the maximum outer diameter of the bullet 10.

  FIG. 3 shows that the bullet 10 has been fired and the front end of the bullet 10 has entered the straight portion 73 of the gun cavity 71. Here, if the space between the armor 21 and the core 11 is clogged at the maximum outer diameter portion of the bullet 10, the diameter of the bullet 10 is difficult to be reduced, and the resistance received when the bullet 10 passes through the barrel 70 is large. Become. As a result, there is a problem that the internal pressure of the gun cavity 71 becomes high and the material constituting the shell 21 is likely to adhere to the gun cavity 71. However, in the present embodiment, as shown in the change from FIG. 3 to FIG. 4, the maximum outer diameter of the armor 21 when the maximum outer diameter cylindrical portion 31 (the armored straight portion 22) passes through the barrel 70. Since the annular gap 31 is formed between the cylindrical portion 31 and the bullet core 11, the bullet 10 (specifically, the maximum outer diameter cylindrical portion 31 of the shell 22) can be deformed inward. Thereby, the resistance received when the bullet 10 passes through the barrel 70 can be reduced, and the problem that the material constituting the shell 21 is likely to adhere to the gun cavity 71 can be suppressed.

  Next, a method for manufacturing the bullet 10 will be described. In order to manufacture the bullet 10, first, as shown in FIG. 5A, a bullet core 11 and a bullet receptor 51 for forming the shell 21 are prepared. The bullet core receptor 51 has a cylindrical structure with one end closed, and includes a straight tube portion 52 and a pointed portion 53 extending from one end of the straight tube portion 52. The inner diameter of the straight tube portion 52 is slightly larger than the outer diameter of the bullet core 11. The straight tube portion 52 is formed longer than the covered straight portion 22, and the sharpened portion 53 is formed in the same shape as the covered head 23.

  Next, as shown by the arrow in FIG. 5A, the bullet 11 is inserted into the bullet receptor 51 from the bullet head 13 side, and as shown in FIG. It is received by the bullet receptor 51. At this time, the bullet head 13 is in contact with the inner surface of the pointed portion 53, and the bullet tail portion 14 is disposed on the front side in the insertion direction from the open end of the bullet receiver 51 (the other end of the straight tube portion 52).

  Next, as shown by an arrow in FIG. 5B, the other end portion of the straight tube portion 52 is caulked, and the armored tail portion 24 and the armored straight portion 22 are formed from the straight tube portion 52. Further, the armored head 23 is formed from the pointed portion 53. Thus, the bullet 10 shown in FIG. 1 is completed.

  Next, the effect of the bullet 10 of this embodiment is demonstrated. In the bullet 10 according to the present embodiment, all the maximum outer diameter cylindrical portions 31 (covered straight portions 22) formed on the shell 21 are annularly formed between the bullet core 11 over the entire region in the axial direction of the bullet 10. Therefore, the maximum outer diameter cylindrical portion 31 can be deformed to the central axis J1 side of the bullet 10 over the entire area of the bullet 10 in the axial direction. Thereby, it is possible to reduce the resistance received when the bullet 10 passes through the barrel 70.

  Further, in the bullet 10 of the present embodiment, the armored head 23 is fitted with the bullet head 13, and the armored tail 24 is fitted with the ballistic tail 14, so that the armored 21 holds the bullet 11. Therefore, it is possible to stabilize the fixation of the bullet core 11.

[Second Embodiment]
Hereinafter, a second embodiment of the present invention will be described with reference to the drawings. As shown in FIG. 6, a bullet 10V for a small firearm (hereinafter simply referred to as “bullet 10V”) of the present embodiment is a modification of the bullet 10 of the first embodiment, and the configuration of the armor 121 Is different from the upper 21 of the first embodiment. The bullet 10V is provided with the same bullet 11 as the bullet 10 of the first embodiment.

  The armor 121 includes an armored straight part 122, an armored head 123, and an armored tail part 124. The armored straight portion 122 has a straight cylindrical shape extending along the central axis J1 of the bullet 10V, and has a constricted portion 125 that is recessed toward the core 11 over the entire circumferential direction. And the annular groove 126 is formed in the outer peripheral surface of the to-be-straightened straight part 122 by the narrow part 125. FIG. The armored head 123 and the armored tail 124 have the same configuration as the armored head 23 and the armored tail 24 of the armor 21 of the first embodiment.

  As shown in FIG. 6, the annular groove 126 of the upper 121 is engaged with the cartridge case 42. Specifically, the open end of the front end cylindrical portion 42A of the cartridge case 42 is bent inward, and the bent portion of the front end cylindrical portion 42A engages with the annular groove 126 in an uneven manner.

  In the bullet 10V, as in the first embodiment, the armored head 123 and the ballistic head 13 are fitted, and the armored tail portion 124 and the ballistic tail portion 14 are fitted. Further, in the bullet 10 </ b> V, the constricted portion 125 of the armored straight portion 122 is fitted with the bullet straight portion 12. That is, in the bullet 10 </ b> V, the bullet 11 is held on the shell 121 at the three locations of the bullet straight part 12, the bullet head 13, and the bullet tail 14.

  As described above, since the constricted portion 125 of the armored straight portion 122 is fitted to the core 11, each of the portion located on the front side and the rear side of the armored straight portion 122 is located on the rear side. In the bullet 10V, it is the maximum outer diameter cylindrical portion 31 that constitutes a portion having the maximum outer diameter. Each of the largest outer diameter cylindrical portions 31 has an annular gap 32 between itself and the core straight portion 12 over the entire axial direction of the bullet 10V.

  The bullet 10V is manufactured as follows. In order to manufacture the bullet 10V, first, as shown in FIG. 7A, a bullet core 11 and a bullet receptor 151 for forming the armor 121 are prepared. The bullet receptor 151 has a structure obtained by deforming the bullet receptor 51 of the first embodiment, and is different from the bullet receptor 51 in that a constricted portion 125 is formed in the straight tube portion 52. .

  Similar to the first embodiment, the bullet 11 is inserted into the bullet receptor 151 (FIG. 7A), and the bullet 11 is received by the bullet receptor 151 (FIG. 7B). ). Next, as shown by an arrow in FIG. 7B, the other end portion of the straight tube portion 52 is caulked, and the armored tail portion 124 and the armored straight portion 122 are formed from the straight tube portion 52. Further, the armored head 123 is formed from the pointed portion 53 of the ballistic receptor 151. Thus, the bullet 10V shown in FIG. 1 is completed.

  As another manufacturing method of the bullet 10V, the bullet receptor 51 of the first embodiment is used in place of the bullet receptor 151, and after the bullet 11 is inserted into the bullet receptor 51, the bullet acceptor is received. The constricted portion 125 may be formed on the body 51.

  Next, the effect of the bullet 10V will be described. In the bullet 10V, similarly to the first embodiment, the maximum outer diameter cylindrical portion 31 can be deformed to the central axis J1 side of the bullet 10 over the entire area in the axial direction of the bullet 10. Thereby, it is possible to reduce the resistance received when the bullet 10 passes through the barrel 70. Further, in the bullet 10V, since the bullet 11 is held at the three locations of the tortoise head 123, the tortoise tail 124, and the constricted portion 125, the fixation of the bullet 11 is stabilized.

[Third Embodiment]
Hereinafter, a third embodiment of the present invention will be described with reference to the drawings. As shown in FIG. 8, a bullet 10W for small arms (hereinafter, simply referred to as “bullet 10W”) of the present embodiment is a modification of the bullet 10V of the second embodiment, and the configuration of the bullet 111 Is different from the bullet 11 of the second embodiment. The bullet 10W has the same armor 121 as the bullet 10V of the second embodiment.

  The bullet core 111 includes two bullet components 111A and a second bullet component 111B made of a member different from the first bullet component 111A. The first core constituent body 111A and the second core constituent body 111B are arranged side by side. 111A of 1st core composition constitutes a part of bullet straight part 112 and bullet head 113, and 2nd article 111B constitutes the remainder of bullet straight part 112, and bullet tail part 114 To do. In the example of FIG. 8, the first core structure 111A is arranged on the front side. Note that the first core structure 111A and the second core structure 111B may be made of the same material, or may be made of different materials.

  In the present embodiment, the constricted portion 125 is fitted with the second core component 111B. Here, the axial length of the second core component 111B is longer than the axial length of the first core component 111A, and the constricted portion 125 is fitted to the front end portion of the second core component 111B. To do. As described above, in this embodiment, the core structure having the longer axial length of the first and second core structures 111A and 111B is held by the shell 122 at two locations. Stabilization of the fixation of the core 111 is achieved. In addition, since the second core component 111B is held by the shell 122 at both ends, the second core component 111B is stably held.

  The bullet 10W of the present embodiment is manufactured in the same manner as the bullet 10V of the second embodiment. At that time, it is preferable to use a bullet receptor 151 having a constricted portion 125.

  According to the bullet 10W of the present embodiment, the same effects as those of the second embodiment can be obtained. Further, in the bullet 10W, since the bullet 111 is composed of the two bullet components 111A and 111B, the fixation of the bullet 111 becomes unstable. However, since the bullet core 111 is held at the three positions of the armored head 123, the armored tail portion 124, and the constricted portion 125, the fixation of the bullet core 111 is stabilized.

[Other Embodiments]
The present invention is not limited to the above-described embodiment. For example, the embodiments described below are also included in the technical scope of the present invention, and various modifications are possible within the scope of the invention other than the following. It can be changed and implemented.

  (1) In the first embodiment, as shown in FIG. 9, an annular groove 126 may be formed on the outer peripheral surface of the upper 21. In this case, the maximum outer diameter cylindrical portion 31 is formed in a portion where the annular groove 126 is not formed in the upper 21. In the example of FIG. 9, each of the portion disposed on the warhead 10 </ b> A side and the portion disposed on the bullet 10 </ b> B side with respect to the annular groove 126 of the armor 21 constitutes the maximum outer diameter cylindrical portion 31.

  (2) In the third embodiment, as shown in FIG. 10 (A), the constricted portion 125 may be fitted with the first core component 111A, or as shown in FIG. 10 (B). In addition, it may be fitted to both the first and second core constituent bodies 111A and 111B.

  (3) In the third embodiment, as shown in FIG. 11, the protrusion 111T is formed on the rear end surface of the first core component 111A, and the protrusion is formed on the front end surface of the second core component 111B. A recess 111H that engages with 111T may be formed. And you may position by making the protrusion 111T of 111 A of 1st core structures, and the recessed part of 2nd core structure 111B engage. In addition, you may form a recessed part in the rear-end surface of 1st core structure 111A, and a protrusion in the front-end surface of 2nd center structure 111B.

  (4) Like the small firearm bullet 10X shown in FIG. 12, the diameter is reduced in a stepped manner between the armored straight portion 22X and the armored tail 24X of the armored 21X on the armored tail 24X side. The step part 21D may be formed. In this case, the bullet core 11X includes only the bullet straight section 12X and the bullet head 13X, and has the truncated cone-shaped bullet tail parts 14 and 114 that are reduced in diameter on the rear side as shown in the above embodiment. do not do. The armored tail portion 24X is fitted to the rear end portion of the core straight portion 12X, and is annular between the portion arranged on the front side of the non-fitting portion of the core straight portion 12X and the armored straight portion 22X. The gap 32 is formed.

10,10V, 10W, 10X bullets for small arms 11,11X, 111 bullets 12,12X, 112 bullet straight parts 13,13X, 113 bullet heads 14,14X, 114 bullet tails 21,21X, 121 22, 22X, 122 Covered straight part 23, 23X, 123 Covered head 24, 24X, 124 Backed part 31 Maximum outer diameter cylindrical part 32-34 Clearance 125 Constricted part

Claims (5)

In bullets for small arms, the outer part of which is covered with armor,
The armor constitutes a portion where the outer diameter is maximum in the bullet for the small firearm, and at least one maximum outer diameter cylindrical portion extending in the axial direction of the bullet for the small firearm is provided,
All the maximum outer diameter cylindrical portions have an annular gap between them and the bullet center over the entire area in the axial direction of the bullet for the small firearm. The bullet for a firearm according to claim 1,
The bullet has a bullet straight portion extending linearly in the axial direction of the bullet for the firearm, a bullet head extending from the bullet straight portion toward the warhead side, and a tapered head; A frustoconical bullet tail extending from the straight portion to the bullet side and being reduced in diameter on the bullet side,
The armor includes a straight cylindrical armor straight portion extending in the axial direction of the bullet for the small arms, and a taper shape extending from the armor straight portion toward the warhead to correspond to the bullet head. It is composed of an upper part and an upper part that extends from the upper part straight part to the tail side and forms a truncated cone shape corresponding to the bullet point.
The armored head is fitted with the ballistic head, and the armored tail is fitted with the ballistic tail, so that the armor holds the ballistic, while the armored straight part is An annular gap is provided between the bullet straight section and the entire bullet direction of the bullet for the small firearm,
All the said maximum outer diameter cylinder parts are provided in the said armored straight part. The bullet for a firearm according to claim 1,
The bullet has a bullet straight portion extending linearly in the axial direction of the bullet for the firearm, a bullet head extending from the bullet straight portion toward the warhead side, and a tapered head; It consists of a bullet tail that extends from the straight part to the bullet side and has a truncated cone shape that is reduced in diameter on the bullet side,
The armor includes a straight cylindrical armor straight portion extending in the axial direction of the bullet for the small arms, and a taper shape extending from the armor straight portion toward the warhead to correspond to the bullet head. The upper part, and the upper part of the upper part that extends from the upper part of the armor straight part to the tail side and has a truncated cone shape corresponding to the center of the tail part,
In the armored straight part, a constricted part that is depressed on the side of the core over the entire circumferential direction is formed,
The armored head is fitted with the ballistic head, the armored tail is fitted with the ballistic tail, and the constricted part is fitted with the ballistic straight part, whereby the armored While holding the bullet, the portion excluding the constricted portion of the armored straight portion has an annular gap between the bullet straight portion over the entire axial direction of the bullet for the small arms,
All the said maximum outer diameter cylinder parts are provided in the site | part except the said constriction part among the said armored straight parts. In the bullet for small arms of Claim 3,
The bullet is configured by arranging a first bullet component and a second bullet component formed of a member different from the first bullet component in the axial direction of the bullet for the firearm. Has been. In the bullet for a small firearm according to any one of claims 2 to 4,
The axial length of the bullet straight part is longer than the axial length of the armored straight part,
An end of the armored tail portion on the side of the armored straight portion has an annular gap between the end of the armored straight portion and the end of the armored head on the armored straight portion side. An annular gap is provided between the straight portion.

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