Polymer ammunition having a MIM primer insert

1. An ammunition cartridge engaged to an injection molded primer insert, the ammunition cartridge having a mouth that opens into a cartridge body, the cartridge body enclosing a propellant chamber and having a coupling region opposite the mouth that is configured to engage the primer insert, the primer insert comprising:

a top surface opposite a bottom surface having a cylindrical coupling element extending away from the bottom surface, the cylindrical coupling element defining an interior surface and an exterior surface;

a primer recess defined through the top surface and in communication with the propellant chamber via a flash hole defined through the bottom surface; and

a flange extending circumferentially about an outer edge of the top surface and one or more protrusions defined in the exterior surface and extending vertically from the flange to the bottom surface;

wherein the coupling region is molded over the cylindrical coupling element, into the interior surface to form a propellant chamber bottom surface, over the exterior surface to cover the one or more protrusions, and to the flange to secure the cartridge body to the primer insert.

2. The ammunition cartridge of claim 1 , further comprising a groove defined in the primer recess circumvolving the flash hole.

3. The ammunition cartridge of claim 2 , wherein the coupling region further extends through the flash hole into the groove to form a polymer coated flash hole.

4. The ammunition cartridge of claim 1 , further comprising a plurality of protrusions defined in the exterior surface and extending from the flange to the bottom surface.

5. The ammunition cartridge of claim 4 , wherein the plurality of protrusions are spaced equally apart from one another about the circumference of the exterior surface.

6. The ammunition cartridge of claim 5 , wherein the plurality of protrusions are configured as triangular prisms.

7. The ammunition cartridge of claim 5 , wherein the plurality of protrusions are configured as semicylinders.

8. The ammunition cartridge of claim 5 , wherein the plurality of protrusions are configured as scallops.

9. The ammunition cartridge of claim 8 , wherein the scalloped protrusions are continuous about the circumference of the exterior surface.

10. The ammunition cartridge of claim 5 , wherein each of the plurality of protrusions has a thickness defined as the distance the protrusion extends outward from the exterior surface.

11. The ammunition cartridge of claim 10 , wherein the thickness of each of the plurality of protrusions is substantially equal to the thickness of each other of the plurality of protrusions such that the thickness of all of the plurality of protrusions is substantially the same.

12. The ammunition cartridge of claim 11 , wherein an outer diameter of the plurality of protrusions is substantially uniform about an outer most circumference of the plurality of protrusions.

13. The ammunition cartridge of claim 12 , wherein the outer diameter of the plurality of protrusions is less than an outer diameter of the flange.

14. The ammunition cartridge of claim 5 , wherein the plurality of protrusions are configured as a combination of triangular prisms, semicylinders and scallops.

15. The ammunition cartridge of claim 1 , further comprising a polymeric projectile end engaged to a polymeric middle body to form the cartridge body.

16. The ammunition cartridge of claim 15 , wherein the polymeric projectile end further comprises a neck linking the mouth to a shoulder region.

17. The ammunition cartridge of claim 16 , wherein the polymeric middle body is connected to the polymeric projectile end at a joint defined below the shoulder region.

18. The ammunition cartridge of claim 15 , wherein the polymer composition making up the polymeric projectile end is different from the polymer composition making up the polymeric middle body.

19. The ammunition cartridge of claim 1 , wherein the injection molded primer insert is composed of stainless steel.

20. The ammunition cartridge of claim 19 , wherein the stainless steel is selected from a group of stainless steel grades, the group consisting of 102, 174, 201, 202, 300, 302, 303, 304, 308, 309, 316, 316L, 316Ti, 321, 405, 408, 409, 410, 415, 416, 416R, 420, 430, 439, 440, 446 or 601-665 grade stainless steel.