Title:
Failsafe training firearms and blank firing adapter therefor
Kind Code:
A1


Abstract:
A modified barrel and vented blank firing adapter for a failsafe firearm suitable for training exercises and in particular for MILES exercises. A preferred modified barrel includes a plurality of blow out apertures spaced about the barrel's longitudinal axis at a location adjacent to a case of a live ammunition cartridge when loaded in the firearm, the blow out apertures venting from the case a substantial portion of combustion gases produced by firing the live ammunition cartridge; a laterally vented blank firing adapter for producing flash at the muzzle upon firing of a blank ammunition cartridge; and a gas port in the barrel sized for admitting to the firearm's cycling gas tube sufficient combustion gases produced by firing the blank ammunition cartridge for implementing cycling operation of the firearm. The laterally vented blank firing adapter preferably includes a cavity for trapping projectile material when a live ammunition cartridge is fired.



Inventors:
La France, Timothy F. (Newport Beach, CA, US)
Application Number:
11/121823
Publication Date:
11/02/2006
Filing Date:
05/02/2005
Assignee:
SUREFIRE, LLC
Primary Class:
International Classes:
F41A21/00
View Patent Images:
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Primary Examiner:
LEE, BENJAMIN P
Attorney, Agent or Firm:
David Weiss (Sherman Oaks, CA, US)
Claims:
I Claim:

1. A barrel assembly for a firearm, comprising: a longitudinal barrel having a muzzle and at least one opening through said barrel at a location adjacent to a case of a live ammunition cartridge when loaded therein, said at least one opening adapted for venting from the case a substantial portion of combustion gases produced by firing the live ammunition cartridge; a muzzle plug secured to the barrel at the muzzle, said muzzle plug including a venting passage configured for venting combustion gases produced by firing a blank ammunition cartridge for producing flash at said muzzle; and a port in said barrel communicating with a cycling gas tube of said firearm, said port sized for admitting to the cycling gas tube sufficient combustion gases produced by firing the blank ammunition cartridge for implementing cycling operation of the firearm.

2. The assembly according to claim 1, wherein: said at least one opening through said barrel comprises a plurality of openings spaced about the longitudinal axis of said barrel.

3. The assembly according to claim 2, wherein: the diameter of said port is greater than the diameter of a gas tube port normally provided in an original equipment barrel for the firearm.

4. The assembly according to claim 2, the firearm being a Colt M4 carbine, wherein: said port has a diameter of approximately 3/32 inch.

5. The assembly according to claim 2, wherein: the size of said port and the configuration of said venting passage of said muzzle plug are related to one another for optimizing cycling of the firearm while producing flash at said muzzle when the blank ammunition cartridge is fired by the firearm.

6. The assembly according to claim 3, wherein: said venting passage of said muzzle plug is configured for laterally venting the combustion gases therefrom.

7. The assembly according to claim 2, wherein: said venting passage of said muzzle plug is configured for laterally venting the combustion gases therefrom.

8. The assembly according to claim 7, wherein: said muzzle plug includes a forward wall for preventing longitudinal projection of projectile material therefrom when the live ammunition cartridge is fired by the firearm.

9. The assembly according to claim 8, wherein: said venting passage of said muzzle plug includes a longitudinal passage open at a rear end of said muzzle plug and at least one passage communicating with said longitudinal passage and open along at least one side of said muzzle plug.

10. The assembly according to claim 9, wherein: said muzzle plug includes a cavity in said forward wall communicating with said longitudinal passage for trapping projectile material projected through said longitudinal passage.

11. The assembly according to claim 9, wherein: said longitudinal passage comprises a longitudinal bore of diameter less than the diameter of a bullet of a live ammunition cartridge fired by the firearm.

12. The assembly according to claim 7, wherein: said venting passage of said muzzle plug includes a longitudinal bore open at a rear end of said muzzle plug and two transverse bores intersecting said longitudinal bore and open along opposite sides of said muzzle plug.

13. The assembly according to claim 12, wherein: each of said transverse bores is perpendicular to said longitudinal bore.

14. The assembly according to claim 13, wherein: said transverse bores are aligned with one another.

15. The assembly according to claim 14, wherein: said muzzle plug includes a cavity communicating with said longitudinal bore for trapping projectile material projected through said longitudinal bore.

16. The assembly according to claim 12, wherein: the diameters of said transverse bores are substantially equal to one another.

17. The assembly according to claim 12, wherein: the aggregate cross-sectional area of said transverse bores is at least four times the cross-sectional area of said longitudinal bore.

18. The assembly according to claim 12, wherein: the diameter of each of said lateral bores is approximately 3.2 millimeters and the diameter of said longitudinal bore is approximately 2.2 millimeters.

19. The assembly according to claim 12, wherein: said assembly includes a flash hider secured to said muzzle and about said muzzle plug, said muzzle plug rotationally positioned such that said transverse bores are aligned with respective slots in said flash hider.

20. A blank firing adapter for a firearm having a barrel with a muzzle, the blank firing adapter comprising: a muzzle plug adapted to be secured to the barrel at the muzzle; and a venting passage in said muzzle plug configured for laterally venting combustion gases produced by a blank ammunition cartridge fired by the firearm.

21. The blank firing adapter according to claim 20, wherein: said muzzle plug includes a forward wall for preventing longitudinal projection of projectile material therefrom when a live ammunition cartridge is fired by the firearm.

22. The blank firing adapter according to claim 21, wherein: said venting passage of said muzzle plug includes a longitudinal passage open at a rear end of said muzzle plug and at least one passage communicating with said longitudinal passage and open along at least one side of said muzzle plug.

23. The blank firing adapter according to claim 22, wherein: said muzzle plug includes a cavity in said forward wall communicating with said longitudinal passage for trapping projectile material projected through said longitudinal passage.

24. The blank firing adapter according to claim 22, wherein: said longitudinal passage comprises a longitudinal bore of diameter less than the diameter of a bullet of a live ammunition cartridge fired by the firearm.

25. The blank firing adapter according to claim 20, wherein: said venting passage of said muzzle plug includes a longitudinal bore open at a rear end of said muzzle plug and two transverse bores intersecting said longitudinal bore and open along opposite sides of said muzzle plug.

26. The blank firing adapter according to claim 25, wherein: each of said transverse bores is perpendicular to said longitudinal bore.

27. The blank firing adapter according to claim 26, wherein: said transverse bores are aligned with one another.

28. The blank firing adapter according to claim 27, wherein: said muzzle plug includes a cavity communicating with said longitudinal bore for trapping projectile material projected through said longitudinal bore.

29. The blank firing adapter according to claim 25, wherein: the diameters of said transverse bores are substantially equal to one another.

30. The blank firing adapter according to claim 25, wherein: the aggregate cross-sectional area of said transverse bores is at least four times the cross-sectional area of said longitudinal bore.

31. The blank firing adapter according to claim 25, wherein: the diameter of each of said lateral bores is approximately 3.2 millimeters and the diameter of said longitudinal bore is approximately 2.2 millimeters.

Description:

BACKGROUND OF THE INVENTION

This invention relates to firearms useful in training exercises for firing blank ammunition cartridges, and more particularly to a barrel assembly and blank firing adapter for such firearms for preventing the projection therefrom of projectile material from inadvertently fired live ammunition cartridges.

The United States Army's Multiple Integrated Laser Engagement System (MILES) is an effective combat simulation training exercise system wherein participants carry a firearm equipped with a laser transmitter and wear a laser detector harness. Whenever a laser pulse is fired which hits a target harness sensor, corresponding signals are transmitted to a control center and a target participant is made instantly aware of the accuracy of the simulated shot by such means as audio alarms and visual displays.

Automatic rifles employed in MILES exercises, such as Colt M4 and M16 carbines, utilize blank ammunition cartridges for simulating the sound and feel of actual combat. The laser transmitter is typically mounted on the firearm's barrel near its muzzle, and detects both percussion and flash generated by the firing of a blank ammunition cartridge to trigger the generation of a laser pulse. The firearms used in MILES typically include an original equipment barrel and a blank firing adapter attached to the muzzle of the barrel. The chamber of such firearms will accept live ammunition cartridges, so that the firing of an accidentally chambered live ammunition cartridge may have catastrophic effects when fired during a MILES exercise.

When using such a firearm with an original equipment barrel and a standard longitudinally vented blank adapter muzzle plug, the combustion gases generated by the firing of a standard blank ammunition cartridge are sufficient to both cycle the firearm and to create sufficient flash at the muzzle for triggering the laser. Specifically, with respect to cycling operation, the minimum gas pressure for cycling the firearm and the pressure of the combustion gases generated by the firing of a blank ammunition cartridge are about the same, i.e. approximately 10,000 psi. The partial occlusion of the muzzle provided by the longitudinally vented blank adapter or muzzle plug sufficiently increases the back pressure of the gases within the barrel when a blank ammunition cartridge is fired, for assuring reliable cycling operation of the firearm.

The pressure generated by the combustion gases produced by the firing a live ammunition cartridge, however, is approximately 50,000 psi or greater. Although the prior art blank firing adapter ostensively serves the further purpose of blocking a fired bullet from an accidentally chambered live ammunition cartridge from being projected from the barrel's muzzle, it has been experienced that the great force generated by the combustion gases of the fired live ammunition cartridge on occasion causes the bullet, or at least fragments of the bullet, as well as fragments of the fractured muzzle plug, to be projected from the barrel's muzzle.

The problem concerning the firing of an accidentally chambered live ammunition cartridge has been addressed in PCT patent application PCT/US 2003/024435, published Feb. 12, 2004 as PCT International Publication No. W02004/013563 A2, by Robert Gee, which application and publication are incorporated herein by reference. Gee's solution was to modify the original equipment barrel by forming apertures through the barrel aligned with the shoulder portion of a chambered live ammunition cartridge. When the chambered live ammunition cartridge is fired, the high pressure of the combustion gases blow out the cartridge case at the barrel apertures, thereby venting the pressure through the barrel apertures so as to deprive the bullet of a substantial amount of its forward driving force.

Although the lower pressure of the combustion gases generated by the firing of a blank ammunition cartridge does not blow out the cartridge case at the barrel apertures, the blank cartridge case normally does not completely seal the barrel apertures so that combustion gases from the fired blank cartridge may leak from the barrel through the apertures. Such leakage tends to deprive combustion gases within the barrel of sufficient back pressure to pressurize the gas tube for proper cycling operation of the firearm. The back pressure may be increased for permitting cycling operation by eliminating the longitudinal orifice in the muzzle plug (i.e., by substantially completely plugging the muzzle 36), thereby permitting cycling operation when firing blank ammunition. Such a solution, however, would prevent the combustion gases from producing flash at the barrel's muzzle to trigger the laser transmitter, rendering the firearm incompatible with MILES exercises.

SUMMARY OF THE INVENTION

The present invention resolves these problems in connection with the modified firearm barrel by providing a vented blank firing adapter at the barrel's muzzle while assuring that the gas port to the gas tube is sufficiently large to effect cycling operation of the firearm. In addition to the blow out apertures, the modified barrel may include enlarging the gas port for increasing the quantity of gas entering the gas tube for reliably effecting cycling operation of the firearm, and further vents sufficient combustion gases at the muzzle for creating sufficient flash for triggering the laser transmitter. Additionally, the preferred embodiment of the blank firing adapter of the present invention is laterally vented and includes provision for trapping projectile material for preventing projection of such projectile material from the barrel's muzzle.

According to one aspect of the present invention, there is provided barrel assembly for a firearm comprising: a longitudinal barrel having a muzzle and at least one opening through the barrel at a location adjacent to a case of a live ammunition cartridge when loaded therein, such at least one opening adapted for venting from the case a substantial portion of combustion gases produced by firing the live ammunition cartridge; a muzzle plug secured to the barrel at the muzzle, the muzzle plug including a venting passage configured for venting combustion gases produced by firing a blank ammunition cartridge for producing flash at the muzzle; and a port in the barrel communicating with a cycling gas tube of the firearm, the port sized for admitting to the gas tube sufficient combustion gases produced by firing the blank ammunition cartridge for implementing cycling operation of the firearm. In the preferred barrel embodiment, the at least one opening in the barrel comprises a plurality of openings or apertures spaced about the longitudinal axis of the barrel.

The diameter of the gas tube port may be greater than the diameter of a gas tube port normally provided with an original equipment barrel for the firearm. For example, an original equipment barrel of a Colt M4 carbine has a diameter of approximately 1/16 inch, whereas a preferred example of a modified barrel assembly according to the present invention includes a 3/32 inch diameter gas tube port for such M4 carbine.

According to another aspect of the present invention, a preferred embodiment of the muzzle plug includes a venting passage configured for laterally venting the combustion gases therefrom. The venting passage includes a longitudinal bore open at a rear end of the muzzle plug, and two transverse bores intersecting the longitudinal bore and open along opposite sides of the muzzle plug. In a preferred muzzle plug embodiment, each of the two transverse bores is perpendicular to the longitudinal bore, and the two transverse bores are aligned with one another.

The muzzle plug preferably includes a cavity communicating with the longitudinal bore for trapping projectile material projected through the longitudinal bore.

The diameters of the two transverse bores are preferably substantially equal to one another, and the aggregate of the cross-sectional areas of the transverse bores is preferably at least four times the cross-sectional area of the longitudinal bore.

BRIEF DESCRIPTION OF THE DRAWINGS

The novel features believed to be characteristic of the present invention, together with further advantages thereof, will be better understood from the following description considered in connection with the accompanying drawings in which preferred embodiments of the invention are illustrated by way of example. It is to be expressly understood, however, that the drawings are for the purpose of illustration and description only and are not intended as a definition of the limits of the invention.

FIG. 1 is side elevation view of an example of a firearm which may be modified for failsafe blank operation according to the present invention;

FIG. 2 is top plan partially exploded view of a barrel and flash suppressor or hider assembly of the firearm example of FIG. 1 including modifications according to a preferred embodiment of the present invention, shown in increased scale;

FIG. 3 is a cross-sectional view of the firearm barrel of FIG. 2 taken along the line 3-3 and viewed in the direction of the appended arrows, shown in further increased scale;

FIG. 4 is a fragment of a cross-section of the firearm barrel of FIG. 2, taken along the line 4-4 of FIG. 3 and viewed in the direction of the appended arrows;

FIG. 5 is a rear view of an example of the front ferrule carried by the firearm barrel of FIG. 2, modified in accordance with an aspect of the present invention;

FIG. 6 is a cross-section of the front ferrule of FIG. 5, taken along the line 6-6 and viewed in the direction of the appended arrows;

FIG. 7 is a side elevation view of a case with exiting bullet of a standard live ammunition cartridge after having been fired by the firearm modified in accordance with FIG. 2;

FIG. 8 is a side view of the fired bullet after striking a vented blank firing adapter or muzzle plug such as of the types shown in FIGS. 9 and 10;

FIG. 9 is a longitudinal cross-section of a firearm muzzle and flash hider with an included prior art vented blank firing adapter or muzzle plug;

FIG. 10 is a longitudinal cross-section of a firearm barrel muzzle and flash hider including the vented blank firing adapter or muzzle plug of FIG. 2 according to an aspect of the present invention;

FIG. 11 is a rear elevation view of the vented blank firing adapter or muzzle plug shown in FIGS. 2 and 10;

FIG. 12 is a side elevation view of the vented blank firing adapter or muzzle plug of FIG. 11;

FIG. 13 is a cross-section of the vented blank firing adapter or muzzle plug of FIG. 12 taken along the line 13-13 and viewed in the direction of the appended arrows; and

FIG. 14 is a side elevation view of the firearm barrel muzzle and flash hider with installed vented blank firing adapter or muzzle plug of FIGS. 2 and 11-13.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Turning first to FIG. 1, there is illustrated an example of a firearm 20, such as a Colt M4 or M16 automatic rifle, the M4 rifle being shown in the example of FIG. 1. Such automatic rifles 20 are well known in the firearms art, and typically include a gas tube 22 communicating with the firearm's barrel 24 at a gas port through the barrel 24 in the vicinity of the firearm's front sight 26, the gas tube 22 extending above the barrel 24 and to the firearm's upper receiver 28. When an ammunition cartridge is fired by the firearm, generated combustion gases are directed through the gas tube 22 to the upper receiver 28 for manipulating the upper receiver to eject the spent cartridge case and to load the new cartridge into the firearm's chamber for effecting semiautomatic or automatic operation of the firearm 20.

M4 and M16 rifles also typically include a handguard (not shown for purposes of clarity of description) about the barrel 24 between and held by the firearm's handguard slip ring 30 and forward support cup or ferrule 32.

When used for MILES exercises, the firearm 20 is provided with a laser transmitter (represented by block 34) mounted to the firearm's barrel 24 forwardly of the front sight 26. The laser transmitter includes sensors (not shown) for detecting the initiation of barrel vibration (or percussion) resulting from the firing of a blank ammunition cartridge and for detecting flash at the firearm's muzzle resulting from the escaping combustion gases burning at the barrel's muzzle 36. A flash suppressor or hider 38 is typically removably secured to the muzzle 36. When the sensors detect both percussion and flash within a predetermined time interval of each other, the laser transmitter is triggered to generate and longitudinally transmit or “fire” a laser pulse.

As used herein, the word “front” or “forward” describes a direction toward (or forwardly of) the muzzle 36 of the barrel 24 (i.e., to the right as shown in FIGS. 1 and 2); “rear” or “rearward” describes the direction opposite the front (i.e., to the left as shown in FIGS. 1 and 2); “above” means vertically above when the firearm 20 is in a firing position with its barrel 24 horizontal; “below” means vertically below when the firearm 20 is in a firing position with its barrel 24 horizontal; “longitudinal” means the direction along or parallel to the longitudinal axis a of the barrel 24; and “transverse” means a direction crossing a longitudinal direction.

When the firearm 20 is employed in MILES exercises, the muzzle 36 is equipped with a venting muzzle plug or blank firing adapter 40 (see FIG. 9), secured to the muzzle 36 by the flash hider 38. The prior art blank adapter muzzle plug 40 includes a longitudinally extending bore or orifice 42 of cross-sectional area for sufficiently increasing the back pressure of the combustion gases generated by the firing of standard blank ammunition cartridges (such as M200 blanks) to cycle the firearm while producing sufficient flash at the muzzle 36 for triggering the laser transmitter 34. An example of such prior art blank firing adapter 40 includes a longitudinal bore 42 having a diameter of approximately 2.03 millimeters. As previously discussed, however, the prior art blank firing adapter 40 does not effectively block projectile material from an accidentally chambered and fired live ammunition cartridge from being projected from the barrel's muzzle 36.

FIGS. 2-6 and 10-14 illustrate modifications to the firearm 20 of FIG. 1 for preventing projectile material from a fired live ammunition cartridge from being propelled through the firearm's muzzle 36 while permitting the firearm 20 (as modified) to operate with blank ammunition cartridges according to the requirements of MILES exercises. One of the modifications to the barrel 24 comprises providing at least one opening through the modified barrel 24′ at a location adjacent to the case of a live ammunition cartridge when loaded therein. In the preferred embodiment, six openings or apertures 44 are machined through the barrel at a location and orientation intersecting (preferably substantially perpendicularly to) the shoulder of the live ammunition cartridge case, the apertures 44 being preferably equally spaced about the longitudinal axis a. This aspect of the modified barrel 24′ is disclosed in the aforementioned Gee patent publication PCT/WO2004/13563 A2, incorporated herein by reference.

As reflected in the Gee patent publication, the position of the apertures 44 against a chambered live ammunition cartridge causes the combustion gases produced by firing the live ammunition cartridge to blow out apertures 46 through and spaced along the shoulder 48 of the case 50 (see FIG. 7) of the live ammunition cartridge 52, emitting a substantial portion of the combustion gases from the case 50 through the apertures 46 and thence through the barrel apertures 44.

Referring to FIGS. 2 and 4-6, these high pressure gases emitted from the barrel apertures 44 are forwardly vented by means of a tubular structure 54 (fabricated, for example, of steel) encircling the barrel 24′ and supported by and between the barrel's rear section 56 and a modified forward ferrule 32′. The principal modifications of the ferrule 32′ include vent holes 58 therethrough for forwardly venting the high pressure gases from the tubular structure 54.

The present invention utilizes a vented blank firing adapter 60 for assuring that flash is produced at the muzzle 36 of the modified barrel 24′ upon firing of a blank ammunition cartridge. As previously discussed, however, a vented blank firing adapter may decrease the back pressure of the combustion gases when firing a blank ammunition cartridge to the extent that insufficient gas is admitted to the gas tube 22 for properly cycling the firearm. To avoid such occurrence, the present invention assures that the gas port 62 (see FIG. 2) communicating with the gas tube 22 is of a diameter adequately large so as to admit a sufficient quantity of gas to the gas tube 22 for reliably effecting proper cycling.

For example, the gas port provided with an original equipment Colt M4 barrel 24 has a diameter of approximately 1/16 inch. In order to assure that sufficient gas is provided to the gas tube 22 for effecting proper cycling operation, the gas port 62 of the modified barrel 24′ of the M4 firearm is enlarged in accordance with the present invention for increasing the quantity of gas to the gas tube 22. In one example, the 1/16 inch diameter gas port of an original equipment M4 barrel was enlarged (or drilled out) to a diameter of 3/32 inch, thereby allowing sufficient gas to enter the gas tube 22 for effecting proper cycling operation of the firearm.

A preferred embodiment of the blank firing adapter 60 according to the present invention is illustrated in FIGS. 2 and 10-14, and may be fabricated of a metal such as stainless steel. The blank firing adapter 60 comprises a generally cylindrical body 64 having a collar 66 fittingly trapped between a rearwardly facing internal shoulder 68 of the flash hider 38 and the front edge 70 of the barrel's muzzle 36 when the flash hider 38 is threadedly installed to the muzzle 36. The installation preferably includes a lock washer 72 about the muzzle 36 and compressed between the rearwardly facing annular edge 74 of the flash hider 38 and a forwardly facing annular shoulder 76 at the rearward vicinity of the muzzle 36. As may be appreciated when comparing such installation (see FIG. 10) to the installation of the prior art vented blank firing adapter 40 (see FIG. 9), installation of the preferred embodiment of the vented blank firing adapter 60 of the present invention does not require any modification to the muzzle 36 of the firearm 20 or to a standard U.S. government issue slotted flash hider 38.

As shown in FIGS. 10 and 14, the cylindrical body 64 of the vented blank firing adapter 60 longitudinally extends forwardly into the flash hider's longitudinally slotted tubular portion 78.

The adapter 60 includes a venting passage configured for laterally venting combustion gases for producing flash. As shown in FIGS. 10-13, the preferred configuration of the venting passage comprises a longitudinal passage or bore 80, extending along the longitudinal axis a and open at the adapter's rear end 82. At least one other passage communicates with the longitudinal passage 80 and is open along at least one side of the adapter's body 64 extending into the flash hider's forward portion 78. In the muzzle plug preferred embodiment 60, the venting passage includes two passages or bores 84 each intersecting the longitudinal bore 80 and open along opposite sides of the adapter body 64, so as to laterally or sidewise vent combustion gases entering the longitudinal bore 80. Preferably, the intersecting or laterally venting bores 84 are each perpendicular to the longitudinal bore 80.

A bullet projected from a fired live ammunition cartridge (for example, the bullet 86 shown in FIG. 7 fired from a cartridge case 50 whether or not provided with the venting apertures 46) may strike the chamfered orifice at the rear end of vented muzzle plug or blank firing adapter 60 or 40 with such force as to deform the front end of the bullet and cause a portion thereof to be extruded into the longitudinal bore 80 or 42, as shown by the deformed bullet 86′ in FIG. 8. A fragment of the extruded bullet portion 88 may separate from the deformed bullet 86′ and be projected along the longitudinal bore 80 or 42. In the case of the prior art vented muzzle plug 40 (FIG. 9), the projectile material 88 will be longitudinally expelled or projected from the forward opening of the longitudinal vent 42, with the potential for causing damage and personal injury to property and persons in the line of fire.

In the case of the preferred embodiment of the laterally vented muzzle plug or blank firing adapter 60 of the present invention, such projection of projectile material 88 is prevented. The body 64 of the laterally vented muzzle plug 60 includes an imperforate forward wall 90 including a cavity 92 longitudinally aligned and communicating with the longitudinal bore 80, for receiving and trapping such projectile material 88.

As represented in FIG. 14, the laterally vented blank firing adapter 60 and the flash hider 38 are installed to the firearm's muzzle 36 such that the side openings of the transverse bores 84 are aligned with respective slots 94 in the flash hider 38, so that the flash sensor of the laser transmitter apparatus 34 is able to “see” a flash event. It is preferred that the openings of the transverse bores 84 be horizontally aligned; i.e., in the preferred embodiment, the laterally open transverse bores 84 are perpendicular to the longitudinal bore 80 and horizontally aligned to each other.

A preferred relationship has been determined between the cross-sectional area of the longitudinal bore 80 and the aggregate cross-sectional area of the two lateral bores 84. Specifically, the aggregate of the cross-sectional areas of the two lateral bores 84 should be greater than or equal to four times the cross-sectional area of the longitudinal bore 80. In an example of the preferred embodiment of the laterally vented muzzle plug 60 for use with a Colt M4 firearm modified as discussed above, the diameter of the longitudinal bore 80 was 2.2 millimeters and the diameter of each of the two lateral bores 84 was approximately 3.2 millimeters, so that the aggregate cross-sectional area of the two lateral bores 84 was approximately 4.2 times the cross-sectional area of the longitudinal bore 80.

Thus, there has been described a preferred embodiment of a modified firearm barrel and vented blank adapter for a failsafe firearm suitable for training exercises and in particular for MILES exercises. Other embodiments of the present invention, and variations of the embodiment presented herein, may be developed without departing from the essential characteristics thereof. Accordingly, the invention should be limited only by the scope of the claims listed below.