Northrop Grumman's Mk266 Mod 1 HEI-T is a U.S.-origin 30x173mm high-explosive incendiary tracer round for Mk44 Bushmaster chain guns and other 30x173mm cannons. Army and manufacturer ammunition documents describe it as a PGU-13-derived service cartridge with a non-self-destruct low-drag fuze, tracer, and shipboard gun-system context.
Mk266 sits in the 30x173 mm Bushmaster ammunition family between earlier PGU-series aircraft ammunition, the self-destruct Mk238 HEI-T/SD, Mk239 training tracer, and programmable Mk310 air-burst round.
| Variant | Configuration | Designation notes |
|---|---|---|
 | High-explosive incendiary tracer with self-destruct | Northrop Grumman distinguishes Mk238 from Mk266 by its self-destruct low-drag fuze, while both are PGU-13-derived HEI-T cartridges in the 30x173 mm suite. |
 | Target-practice tracer | Northrop Grumman identifies Mk239 TP-T as a training round ballistically matched to Mk266 HEI-T and Mk238 HEI-T/SD. |
| Programmable air-burst munition with tracer | Mk310 is another 30x173 mm Bushmaster-family round, but it uses programmable airburst, point-detonate, and delay modes rather than Mk266's conventional low-drag HEI-T configuration. |
Mk266 is one named service cartridge within the broader 30x173 mm Bushmaster ammunition suite.
| Compatible item | Item type | Compatibility evidence |
|---|---|---|
 | 30 mm ammunition family | Northrop Grumman's 30x173 mm family includes Mk266 HEI-T alongside Mk238 HEI-T/SD, Mk239 TP-T, Mk310 PABM-T, and other Bushmaster-compatible cartridges. Sources: Northrop Grumman 30x173mm Full Ammo Suite Brochure, 30x173mm Bushmaster Chain Guns |
Northrop Grumman and Army ammunition documents place Mk266 Mod 1 HEI-T in the 30x173mm Bushmaster family and in Mk44-class gun service.
| Firing weapon | Weapon type | Firing evidence |
|---|---|---|
 | 30 mm naval gun mount | The DS30M family uses the Mk 44 cannon, and the Mk266 HEI-T is described as a 30x173mm round usable in Mk44 or other 30x173mm cannons. Sources: MK 266 Mod 1 HEI-T - PEO Ammunition, Northrop Grumman 30x173mm Full Ammo Suite Brochure, MSI-DS SEAHAWK DS A2 |
 | 30 mm naval gun weapon system | The U.S. Navy identifies the MK 46 Mod 2 as a remotely operated Mk 44 Mod 2 30 mm gun system with a 400-round magazine, while Northrop Grumman identifies Mk266 as a 30x173 mm round used in Mk44 or other 30x173 mm cannons. Sources: MK 266 Mod 1 HEI-T - PEO Ammunition, MK 46 - 30 mm Gun Weapon System, Northrop Grumman 30x173mm Full Ammo Suite Brochure |
Mk266 is best read as a service round inside a shared 30x173 mm Bushmaster ecosystem rather than as an independent weapon system. The cited ammunition documents separate three useful reader cues: cartridge dimensions, fuze behavior, and firing-gun compatibility.
Mk266 uses the non-self-destruct low-drag fuze branch, while Mk238 HEI-T/SD uses the self-destruct branch.
Mk239 TP-T is documented as a ballistic training match for Mk266 HEI-T and Mk238 HEI-T/SD.
Northrop Grumman places Mk266 in Mk44 and other 30x173 mm cannon use, which supports links to Mk44-class shipboard mounts.
Sources: Northrop Grumman 30x173mm Full Ammo Suite Brochure; MK 266 Mod 1 HEI-T - PEO Ammunition; MK 46 - 30 mm Gun Weapon System; MSI-DS SEAHAWK DS A2.
The FY 2017 Army ammunition justification book requested $6.000 million for 0.048 million Mk266 HEI-T cartridges.
Sources: FY 2017 Army Ammunition Justification Book
The 2025 PEO Ammunition portfolio described the 30 mm Mk266 Mod 1 HEI-T cartridge with 30x173 mm data, low-drag fuze details, and shipboard gun-system context.
Sources: MK 266 Mod 1 HEI-T - PEO Ammunition
