Upper receiver and JIG for takedown pin set screws

Abstract

A gun with a two part receiver having no gap between the upper and lower receivers of the two part receiver. The upper receiver has a front lug and a rear lug. To join the upper receiver with the lower receiver, a front pin inserts through the lower receiver and the front lug, and a rear pin inserts through the lower receiver and the rear lug. A threaded hole in extends upwardly into the front lug such that a set screw can tighten against the front pin to draw the upper receiver downardly into the lower receiver. Similarly, a threaded hole extends upwardly into the rear lug such that a set screw can tighten against the rear pin and draw the rear of the upper receiver down into the lower receiver. In another aspect, a jig for drilling and tapping the threaded holes is provided.

Description

CROSS-REFERENCE TO RELATED APPLICATIONS

[0001] This Application claims priority to and hereby incorporates by reference in its entirety U.S. Provisional Patent Application Ser. No. 63 / 611,616 entitled “UPPER RECEIVER AND JIG FOR TAKEDOWN PIN SET SCREWS” filed Dec. 18, 2023.

[0002] A portion of the disclosure of this patent document contains material that is subject to copyright protection. The copyright owner has no objection to the reproduction of the patent document or the patent disclosure, as it appears in the U.S. Patent and Trademark Office patent file or records, but otherwise reserves all copyright rights whatsoever.FEDERALLY SPONSORED RESEARCH AND DEVELOPMENT

[0003] Not ApplicableBACKGROUND OF THE INVENTION

[0004] The present invention relates generally to guns with two part receivers (i.e., an upper and a lower receiver). More particularly, this invention pertains to systems and methods for reducing play between the two parts of the receiver.

[0005] In two part receiver type gun platforms, machining tolerances and variances cause the upper and lower receiver to not fit tightly with one another when the upper receiver is secured to the lower receiver (e.g., by front and rear takedown pins). Referring to PRIOR ART FIG. 3, there can even be a gap 200 between the upper receiver 202 and lower receiver 105. Play between the upper and lower receiver can cause poor accuracy, operational malfunctions, can allow the gun to rattle when in transit, and can be aesthetically unpleasing.

[0006] Several different methods of eliminating play or slop between the upper and lower receivers have been tried in the past. In one implementation, a wedge pushes the bottom of the rear lug of the upper receiver up which prevents the upper and lower receiver from rattling against one another. However, the gap between the upper and lower receiver remains which throws parts of the action off center. That is, the bolt carrier group is misaligned from the buffer tube due to the wedge holding the upper receiver out of alignment with the lower receiver and the buffer tube in the lower receiver. Another implementation similarly uses a set screw to push the rear lug of the upper receiver up and away from the lower receiver which results in the same permanent gap and misalignment issues as the wedge solution. Yet another implementation uses shims fitted about the lugs of the upper receiver to force the upper and lower receivers apart. Again, this results in a permanent gap between the upper and lower receiver and misalignment of the upper and lower receivers.SUMMARY OF THE INVENTION

[0007] Aspects of the present invention provide a gun with a two part receiver having no gap between the upper and lower receivers of the two part receiver. The upper receiver has a front lug and a rear lug. To join the upper receiver with the lower receiver, a front pin inserts through the lower receiver and the front lug, and a rear pin inserts through the lower receiver and the rear lug. A threaded hole in extends upwardly and rearwardly into the front lug such that a set screw can tighten against the front pin to draw the upper receiver downardly into the lower receiver. Similarly, a threaded hole extends upwardly and rearwardly into the rear lug such that a set screw can tighten against the rear pin and draw the rear of the upper receiver down into the lower receiver. In another aspect, a jig for drilling the threaded holes and a tap for tapping the threaded holes are provided.

[0008] In another aspect, an upper receiver for a gun extends along a longitudinal axis. The upper receiver includes a lug, a threaded hole through the lug, and a set screw. The lug has a lateral hole therethrough for receiving a pin. The lug is configured to attach the upper receiver to a complementary lower receiver of the gun via the pin when the pin is inserted laterally through the lateral hole through the lug and a corresponding lateral hole in the lower receiver. The set screw is configured to screw into the threaded hole and bias the pin against an interior surface of the lateral hole through the lug.

[0009] In another aspect, a gun includes a lower receiver and an upper receiver. The lower receiver includes a trigger group and a stock. The upper receiver extends along a longitudinal axis. The upper receiver includes a barrel, a lug, a threaded hole through the lug, and a set screw. The lug has a lateral hole therethrough for receiving a pin. The lug is configured to attach the upper receiver to a complementary lower receiver of the gun via the pin when the pin is inserted laterally through the lateral hole through the lug and a corresponding lateral hole in the lower receiver. The set screw is configured to screw into the threaded hole and bias the pin against an interior surface of the lateral hole through the lug.

[0010] In yet another aspect, a jig for an upper receiver of a gun is provided. The upper receiver is configured to extend along the longitudinal axis and connect to a lower receiver of the gun via a front takedown pin and a rear takedown pin. The front takedown pin is configured to extend laterally through a lateral hole through the front lug and a corresponding lateral hole in the lower receiver when the gun is assembled. The rear takedown pin is configured to extend through a lateral hole through the rear lug and a corresponding hole in the lower receiver when the gun is assembled. The jig includes a body. The body includes a pair of opposing sidewalls and a base. The pair of opposing sidewalls includes a front pair of holes through the opposing sidewalls corresponding to the lateral hole through the front lug of the upper receiver such that a front takedown pin inserted through the front pair of holes and the lateral hole through the front lug of the upper receiver holds the body in alignment with the upper receiver. The pair of opposing sidewalls also includes a rear pair of holes through the opposing sidewalls corresponding to the lateral hole through the rear lug of the upper receiver such that a rear takedown pin inserted through the rear pair of holes and the lateral hole through the rear lug of the upper receiver holds the body in alignment with the upper receiver. The base is configured to support the pair of opposing sidewalls and space the pair of opposing sidewalls from one another such that the sidewalls extend generally upwardly from the base when the jig is in an upright position. The base includes a front hole and a rear hole. The front hole through the base is configured to receive a drill bit forward of a center of the front pair of lateral holes of the pair of opposing sidewalls such that the drill bit extends upwardly and rearwardly through the front lug into the lateral hole through the front lug when the upper receiver is received in the jig in an upright position. The rear hole through the base is configured to receive the drill bit forward of a center of the rear pair of lateral holes of the pair of opposing sidewalls such that the drill bit extends upwardly and rearwardly through the rear lug into the lateral hole through the rear lug when the upper receivers received in the jig in the upright position.BRIEF DESCRIPTION OF DRAWINGS

[0011] FIG. 1 is a side perspective view of a gun according to one embodiment of the invention.

[0012] FIG. 2 is a partially exploded side perspective view of the gun of FIG. 1.

[0013] FIG. 3 is a partial side perspective view of a prior art gun.

[0014] FIG. 4 is an elevated isometric view of the gun of FIG. 1 showing a front lug with a set screw.

[0015] FIG. 5 is an elevated isometric view of the upper receiver of the gun of FIG. 1.

[0016] FIG. 6 is an elevated isometric view of a rear lug of the upper receiver of the gun of FIG. 1.

[0017] FIG. 7 is an isometric view of a side cut away of the gun of FIG. 1 showing the front pin and front set screw in the upper receiver.

[0018] FIG. 8 is a side cutaway view of the gun of FIG. 1 showing the rear lug and rear set screw of the upper receiver.

[0019] FIG. 9 is a side perspective view of a jig receiving an upper receiver according to another embodiment of the invention.

[0020] FIG. 10 is an elevated isometric view of the jig of FIG. 9.

[0021] FIG. 11 is an isometric view of the jig of FIG. 9.

[0022] FIG. 12 is an elevated isometric view of a tap and the rear lug of the upper receiver shown in FIG. 9 after having been drilled with the jig of FIGS. 9-11.

[0023] Reference will now be made in detail to optional embodiments of the invention, examples of which are illustrated in accompanying drawings. Whenever possible, the same reference numbers are used in the drawing and in the description referring to the same or like parts.DETAILED DESCRIPTION OF THE INVENTION

[0024] While the making and using of various embodiments of the present invention are discussed in detail below, it should be appreciated that the present invention provides many applicable inventive concepts that can be embodied in a wide variety of specific contexts. The specific embodiments discussed herein are merely illustrative of specific ways to make and use the invention and do not delimit the scope of the invention.

[0025] To facilitate the understanding of the embodiments described herein, a number of terms are defined below. The terms defined herein have meanings as commonly understood by a person of ordinary skill in the areas relevant to the present invention. Terms such as “a,”“an,” and “the” are not intended to refer to only a singular entity, but rather include the general class of which a specific example may be used for illustration. The terminology herein is used to describe specific embodiments of the invention, but their usage does not delimit the invention, except as set forth in the claims.

[0026] As described herein, an upright position is considered to be the position of apparatus components while in proper operation or in a natural resting position as described herein. As used herein, the upright or vertical position of a gun or firearm is when assembled and held by a shooter, ready to shoot (i.e., fire or discharge), with the bore of the barrel of the gun extending generally horizontally or level along a longitudinal axis and the trigger extending generally downward. Vertical, horizontal, above, below, side, top, bottom and other orientation terms are described with respect to this upright position during operation unless otherwise specified. The term “when” is used to specify orientation for relative positions of components, not as a temporal limitation of the claims or apparatus described and claimed herein unless otherwise specified. The terms “above”, “below”, “over”, and “under” mean “having an elevation or vertical height greater or lesser than” and are not intended to imply that one object or component is directly over or under another object or component.

[0027] The phrase “in one embodiment,” as used herein does not necessarily refer to the same embodiment, although it may. Conditional language used herein, such as, among others, “can,”“might,”“may,”“e.g.,” and the like, unless specifically stated otherwise, or otherwise understood within the context as used, is generally intended to convey that certain embodiments include, while other embodiments do not include, certain features, elements and / or states. Thus, such conditional language is not generally intended to imply that features, elements and / or states are in any way required for one or more embodiments or that one or more embodiments necessarily include logic for deciding, with or without operator input or prompting, whether these features, elements and / or states are included or are to be performed in any particular embodiment.

[0028] The terms “coupled” and “connected” mean at least either a direct electrical or mechanical connection between the connected items or an indirect connection through one or more passive or active intermediary devices.

[0029] Referring now to FIGS. 1, 2, and 4-8, an upper receiver 103 and gun 100 including set screws 155, 355 which eliminate the gap 200 between upper receiver 103 and lower receiver 105 from the gun 100 are shown. It is contemplated within the scope of the claims that only one set screw 155 may be used in one lug 150 of the upper receiver 103 of the gun 100. The gun 100 includes lower receiver 105 and upper receiver 103. The lower receiver 104 includes trigger group 131 and stock 133. The upper receiver 103 includes barrel 130 extending along longitudinal axis 101 as well as a front lug 150. The front lug 150 has a lateral hole 151 therethrough for receiving front pin 159. In one embodiment, the front pin 159 is a takedown pin (e.g., a captured pin with a detent system to retain the pin 159 in place). The front lug 150 is configured to attach the upper receiver 103 to the complementary lower receiver 105 of the gun 100 via the front pin 159 when the front pin 159 is inserted laterally through the lateral hole 151 through the front lug 150 and a corresponding laterally hole 161 (e.g., pair of lateral holes 161) in the lower receiver 105. The front lug 150 has a threaded hole 153 therethrough, and the front set screw 155 is configured to screw into the threaded hole 153 and bias the pin 159 against an interior surface 157 of the lateral hole 151 through the front lug 150 when the gun 100 is assembled. In one embodiment, the threaded hole 153 in the front lug 150 extends along a first line at an angle of between fifteen and 75 degrees to the longitudinal axis 101 such that the front set screw 155 biases the front pin 159 upward with respect to the upper receiver 103. In one embodiment, the threaded hole 153 extends perpendicularly to the front pin 159 when the front pin 159 is inserted laterally through the lateral hole 151 in the front lug 150.

[0030] In one embodiment, the rear lug 350 of the upper receiver 103 includes a rear set screw 355 similar to the front lug 150 and front set screw 155 configured to pull a rear end of the upper receiver 103 down with respect to the lower receiver 105 (by pushing the rear pin 359 upward within the rear lug 350). The rear lug 350 has a lateral hole 351 therethrough for receiving the rear pin 359. The rear lug 350 is configured to attach the upper receiver 103 to the lower receiver 105 via the rear pin 359 when the rear pin 359 is inserted laterally through the lateral hole in the rear lug 350 and a corresponding lateral hole 361 (e.g., pair of lateral holes) through the lower receiver 105. The rear lug 350 has a threaded hole 353 therethrough. A rear set screw 355 is configured to screw or thread into the threaded hole 353 in the rear lug 350 and bias the rear pin 350 against an internal surface 357 of the lateral hole 351 through the rear lug 350 when the gun 100 is assembled. In one embodiment, the threaded hole 353 in the front lug 350 extends along a first line at an angle of between fifteen and 7f5 degrees to the longitudinal axis 101 such that the rear set screw 355 biases the rear pin 359 upward with respect to the upper receiver 103. In one embodiment, the rear pin 359 is a takedown pin. In one embodiment, the rear threaded hole 353 extends perpendicularly to the rear pin 359 when the rear pin 359 is inserted laterally through the lateral hole 351 in the rear lug 350.

[0031] Referring to FIGS. 1, 2, and 4-12 (especially FIGS. 9-12), in one embodiment, a jig 500 including a body 501. The purpose of the jig 500 is to align, drill, and tap the threaded holes 153, 353 in the front and rear lugs of the prior art upper receiver 202 to produce the upper receiver 103 having the front threaded hole 153 and rear threaded hole 353. The body 501 is configured to receive the front lug 150 and the rear lug 350 of the upper receiver 103 therein. The body 501 includes a pair of opposing sidewalls 503 and a base 509. The pair of opposing sidewalls 503 includes a front pair of holes 505 through the opposing sidewalls 503 corresponding to the lateral hole 151 through the front lug 150 of the upper receiver 103 such that the front takedown pin 159 inserted through the front pair of holes 505 and the lateral hole 151 through the front lug 150 of the upper receiver 103 holds the body 501 in alignment with the upper receiver 103. The pair of opposing sidewalls 503 includes a rear pair of holes through the opposing sidewalls 503 corresponding to the lateral hole 351 through the rear lug 350 of the upper receiver 103 such that a rear takedown pin 359 inserted through the rear pair of holes 507 and the lateral hole 351 through the rear lug 350 of the upper receiver 103 holds the body 501 in alignment with the upper receiver 103. The base 509 is configured to support the pair of opposing sidewalls 503 and space the pair of opposing sidewalls 503 from one another such that the sidewalls 503 extend generally upwardly from the base when the Jake is in an upright position. The base 509 includes a front hole 511 through the base 509 configured to receive a drill bit 515 forward of the center of the front pair of lateral holes 505 of the pair of opposing sidewalls 503 such that the drill bit 515 extends upwardly through the front lug 150 into the lateral hole 151 through the front lug 150 when the upper receiver 103 is received in the jig 500 in an upright position. The base 509 also includes a rear hole 513 through the base 509 configured to receive the drill bit 515 forward of a center of the rear pair of lateral holes 507 of the pair of opposing sidewalls 503 such that the drill bit 515 extends upwardly through the rear lug 350 into the lateral hole 351 through the rear lug 350 when the upper receiver 103 is received in the jig 500 in the upright position. In one embodiment, the jig 500 is a kit including the body 501, drill bit 515, the front takedown pin 159, the rear takedown pin 359, the pair of set screws 155, 355, and a tap 517. The tap 517 has a diameter thread pattern corresponding to the drill bit 515 and the pair of setscrews 155, 355. In one embodiment, the body 501 is formed of steel such that the body 501 is harder than the upper receiver 103. The upper receiver 103 will typically be aluminum, but may be another material. In one embodiment, the base 509 and each opposing sidewall 503 is generally planar. In one embodiment, the pair of opposing sidewalls 503 are closer together at the pair of rear holes 507 than at the pair of front holes 511 such that an internal width of the body 501 matches an external width of the front lug 150 and rear lug 350.

[0032] Although generally shown and described herein with the threaded holes 153, 353 extending upward and rearward, it is contemplated within the scope of the claims that the threaded holes 153 and 353 may extend upwardly and forward such that the set screws 155, 355 bias the upper receiver 103 downward and rearward with respect to the lower receiver 105 instead of downward and forward.

[0033] This written description uses examples to disclose the invention and also to enable any person skilled in the art to practice the invention, including making and using any devices or systems and performing any incorporated methods. The patentable scope of the invention is defined by the claims, and may include other examples that occur to those skilled in the art. Such other examples are intended to be within the scope of the claims if they have structural elements that do not differ from the literal language of the claims, or if they include equivalent structural elements with insubstantial differences from the literal languages of the claims.

[0034] It will be understood that the particular embodiments described herein are shown by way of illustration and not as limitations of the invention. The principal features of this invention may be employed in various embodiments without departing from the scope of the invention. Those of ordinary skill in the art will recognize numerous equivalents to the specific procedures described herein. Such equivalents are considered to be within the scope of this invention and are covered by the claims.

[0035] All of the compositions and / or methods disclosed and claimed herein may be made and / or executed without undue experimentation in light of the present disclosure. While the compositions and methods of this invention have been described in terms of the embodiments included herein, it will be apparent to those of ordinary skill in the art that variations may be applied to the compositions and / or methods and in the steps or in the sequence of steps of the method described herein without departing from the concept, spirit, and scope of the invention. All such similar substitutes and modifications apparent to those skilled in the art are deemed to be within the spirit, scope, and concept of the invention as defined by the appended claims.

[0036] Thus, although there have been described particular embodiments of the present invention, it is not intended that such references be construed as limitations upon the scope of this invention except as set forth in the following claims.

Claims

1. An upper receiver for a gun, said upper receiver extending along a longitudinal axis, said upper receiver comprising:a lug having a lateral hole therethrough for receiving a pin, wherein the lug is configured to attach the upper receiver to a complementary lower receiver of the gun via the pin when the pin is inserted laterally through the lateral hole through the lug and a corresponding lateral hole in the lower receiver;a threaded hole through the lug; anda set screw configured to screw into the threaded hole and bias the pin against an interior surface of the lateral hole through the lug.

2. The upper receiver of claim 1, wherein:the interior surface of the lateral hole through the lug is opposite the threaded hole through the lug.

3. The upper receiver of claim 1, wherein:the set screw is a hex key machine screw.

4. The upper receiver of claim 1, wherein:the threaded hole extends along a line at an angle of between fifteen and 75 degrees to the longitudinal axis such that the set screw biases the pin rearward and upward with respect to the upper receiver.

5. The upper receiver of claim 1, wherein:the set screw is configured to bias the pin against an inside surface of the lateral hole through the lower receiver when the upper receiver and lower receiver are assembled to form the gun.

6. The upper receiver of claim 1, wherein:the lateral hole in the lower receiver is one of a pair of lateral holes in the lower receiver corresponding to the lateral hole of the lug of the upper receiver, and the lug is configured to attach the upper receiver to the complementary lower receiver of the gun via the pin when the pin is inserted laterally through the lug and the corresponding pair of lateral holes in the lower receiver with the lateral hole in the lug of the upper receiver between the pair of lateral holes in the lower receiver.

7. The upper receiver of claim 1, wherein:the threaded hole extends perpendicularly to the pin when the pin is inserted laterally through the lateral hole in the lug.

8. The upper receiver of claim 1, wherein:the pin is a takedown pin.

9. The upper receiver of claim 1, wherein:the lug is a front lug of the upper receiver;the pin is a front pin;the set screw is a front set screw;the upper receiver further comprises a rear lug;the rear lug has a lateral hole therethrough for receiving a rear pin, wherein the rear lug is configured to attach the upper receiver to the complementary lower receiver of the gun via the rear pin when the rear pin is inserted laterally through the lateral hole through the rear lug and a corresponding lateral hole in the lower receiver;the upper receiver further comprises a threaded hole through the rear lug; andthe upper receiver further comprises a rear set screw configured to screw into the threaded hole in the rear lug and bias the rear pin against an interior surface of the lateral hole through the rear lug when the gun is assembled.

10. The upper receiver of claim 1, wherein:the lug is a front lug of the upper receiver;the pin is a front pin;the set screw is a front set screw;the upper receiver further comprises a rear lug;the rear lug has a lateral hole therethrough for receiving a rear pin, wherein the rear lug is configured to attach the upper receiver to the complementary lower receiver of the gun via the rear pin when the rear pin is inserted laterally through the lateral hole through the rear lug and a corresponding lateral hole in the lower receiver;the upper receiver further comprises a threaded hole through the rear lug;the upper receiver further comprises a rear set screw configured to screw into the threaded hole in the rear lug and bias the rear pin against an interior surface of the lateral hole through the rear lug when the gun is assembled;the threaded hole in the front lug extends along a first line at an angle of between fifteen and 75 degrees to the longitudinal axis such that the front set screw biases the front pin upward with respect to the upper receiver; andthe threaded hole in the rear lug extends along a second line at an angle of between fifteen and 75 degrees to the longitudinal axis such that the rear set screw biases the rear pin upward with respect to the upper receiver.

11. A gun comprising:a lower receiver comprising:a trigger group; anda stock;an upper receiver extending along a longitutindal axis, said upper receiver comprising:a barrel extending along the longitudinal axis;a lug having a lateral hole therethrough for receiving a pin, wherein the lug is configured to attach the upper receiver to a complementary lower receiver of the gun via the pin when the pin is inserted laterally through the lateral hole through the lug and a corresponding lateral hole in the lower receiver;a threaded hole through the lug; anda set screw configured to screw into the threaded hole and bias the pin against an interior surface of the lateral hole through the lug when the gun is assembled.

12. The gun of claim 11, wherein:the lug is a front lug of the upper receiver;the pin is a front pin;the set screw is a front set screw;the upper receiver further comprises a rear lug;the rear lug has a lateral hole therethrough for receiving a rear pin, wherein the rear lug is configured to attach the upper receiver to the complementary lower receiver of the gun via the rear pin when the rear pin is inserted laterally through the lateral hole through the rear lug and a corresponding lateral hole in the lower receiver;the upper receiver further comprises a threaded hole through the rear lug;the upper receiver further comprises a rear set screw configured to screw into the threaded hole in the rear lug and bias the rear pin against an interior surface of the lateral hole through the rear lug when the gun is assembled;the threaded hole in the front lug extends along a first line at an angle of between fifteen and 75 degrees to the longitudinal axis such that the front set screw biases the front pin upward with respect to the upper receiver; andthe threaded hole in the rear lug extends along a second line at an angle of between fifteen and 75 degrees to the longitudinal axis such that the rear set screw biases the rear pin upward with respect to the upper receiver.

13. A jig for an upper receiver of a gun, said upper receiver configured to extend along a longitudinal axis and connect to a lower receiver of the gun via a front takedown pin and a rear takedown pin, the front takedown pin configured to extend laterally through a lateral hole through the front lug and a corresponding lateral hole in the lower receiver when the gun is assembled, the rear takedown pin configured to extend through a lateral hole through the rear lug and a corresponding hole in the lower receiver when the gun is assembled, said jig comprising:a body configured to receive the front lug and the rear lug of the upper receiver, said body comprising:a pair of opposing sidewalls comprising:a front pair of holes through the opposing sidewalls corresponding to the lateral hole through the front lug of the upper receiver such that a front takedown pin inserted through the front pair of holes and the lateral hole through the front lug of the upper receiver holds the body in alignment with the upper receiver;a rear pair of holes through the opposing sidewalls corresponding to the lateral hole through the rear lug of the upper receiver such that a rear takedown pin inserted through the rear pair of holes and the lateral hole through the rear lug of the upper receiver holds the body in alignment with the upper receiver; anda base configured to support the pair of opposing sidewalls and space the pair of opposing sidewalls from one another such that the sidewalls extend generally upwardly from the base when the jig is in an upright position, said base comprising:a front hole through the base configured to receive a drill bit forward of a center of the front pair of lateral holes of the pair of opposing sidewalls such that the drill bit drills extends upwardly through the front lug into the lateral hole through the front lug when the upper receiver is received in the jig in an upright position; anda rear hole through the base configured to receive the drill bit forward of a center of the rear pair of lateral holes of the pair of opposing sidewalls such that the drill bit drills extends upwardly through the rear lug into the lateral hole through the rear lug when the upper receiver is received in the jig in the upright position.

14. The jig of claim 13 further comprising:the drill bit;the front takedown pin;the rear takedown pin;a pair of set screws; anda tap having a diameter and thread pattern corresponding to the drill bit and the pair of set screws.

15. The jig of claim 13, wherein:the body is formed of steel such that the body is a harder material than the upper receiver.

16. The jig of claim 13, wherein:the base of the jig is generally planar.

17. The jig of claim 13, wherein:each sidewall of the pair of opposing sidewalls is generally planar.

18. The jig of claim 13, wherein:the pair of opposing sidewalls are closer together at the pair of rear holes than at the pair of front holes.

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