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CMMG
Building a 9mm or .22LR AR-15 platform rifle? The CMMG Mk4 AR-15 ejection port cover kit with gas deflector is a straightforward upgrade that keeps hot gas, powder residue, and spent brass spray off your face and out of your shooting environment. Drop-in installation means no gunsmithing required—it's the kind of practical enhancement that matters once you've run a few magazines downrange and felt that thermal and particulate blow-back. This kit is purpose-built for smaller-caliber AR builds where gas dynamics differ significantly from full-power rifle rounds.
The kit includes everything you need: a steel ejection port cover, roll pin, detent spring, and polymer gas deflector. The cover arrives with detent hardware already installed, so assembly is genuinely minimal. The gas deflector is engineered specifically to redirect gas away from the shooter without adding bulk or weight to the upper receiver. A critical note: the polymer deflector is rated for 9mm and .22LR only. Using it on anything larger than .22LR will compromise the material and defeat the purpose, so this kit is not a universal solution across caliber families.
If you run subsonic ammunition, shoot indoors, or simply want to reduce the grit and heat that cycles back toward your face on semi-auto runs, this is a no-nonsense addition. The steel cover itself protects the ejection port from dust and debris when the rifle is at rest, while the gas deflector actively works during firing. Installation is within reach of any AR builder comfortable with basic takedown and reassembly.
| Brand | CMMG |
| Model | Mk4 AR-15 Ejection Port Cover Kit |
| Caliber Compatibility | 9mm, .22LR |
| Cover Material | Steel |
| Deflector Material | Polymer |
| Includes | Steel cover, roll pin, detent spring, polymer gas deflector |
No. This kit is designed specifically for 9mm and .22LR builds. The polymer gas deflector will be damaged if used with larger calibers. For full-power rifle cartridges, you'll need a different solution.
No. It's a drop-in kit. The detent and spring are already installed on the cover itself, so you're mainly inserting the assembly into the ejection port and securing it with the roll pin. Standard AR takedown and reassembly skills are all you need.
The polymer gas deflector redirects hot gas and powder residue that normally cycles back out the ejection port, keeping that thermal and particulate cloud away from your face. It's especially noticeable during high-round-count sessions and when shooting indoors.
The kit includes a steel ejection port cover (with detent hardware pre-installed), a polymer gas deflector, a roll pin, and a detent spring. Everything needed for a complete installation.
CMMG
Building a 9mm or .22LR AR-15 platform rifle? The CMMG Mk4 AR-15 ejection port cover kit with gas deflector is a straightforward upgrade that keeps hot gas, powder residue, and spent brass spray off your face and out of your shooting environment. Drop-in installation means no gunsmithing required—it's the kind of practical enhancement that matters once you've run a few magazines downrange and felt that thermal and particulate blow-back. This kit is purpose-built for smaller-caliber AR builds where gas dynamics differ significantly from full-power rifle rounds.
The kit includes everything you need: a steel ejection port cover, roll pin, detent spring, and polymer gas deflector. The cover arrives with detent hardware already installed, so assembly is genuinely minimal. The gas deflector is engineered specifically to redirect gas away from the shooter without adding bulk or weight to the upper receiver. A critical note: the polymer deflector is rated for 9mm and .22LR only. Using it on anything larger than .22LR will compromise the material and defeat the purpose, so this kit is not a universal solution across caliber families.
If you run subsonic ammunition, shoot indoors, or simply want to reduce the grit and heat that cycles back toward your face on semi-auto runs, this is a no-nonsense addition. The steel cover itself protects the ejection port from dust and debris when the rifle is at rest, while the gas deflector actively works during firing. Installation is within reach of any AR builder comfortable with basic takedown and reassembly.
| Brand | CMMG |
| Model | Mk4 AR-15 Ejection Port Cover Kit |
| Caliber Compatibility | 9mm, .22LR |
| Cover Material | Steel |
| Deflector Material | Polymer |
| Includes | Steel cover, roll pin, detent spring, polymer gas deflector |
No. This kit is designed specifically for 9mm and .22LR builds. The polymer gas deflector will be damaged if used with larger calibers. For full-power rifle cartridges, you'll need a different solution.
No. It's a drop-in kit. The detent and spring are already installed on the cover itself, so you're mainly inserting the assembly into the ejection port and securing it with the roll pin. Standard AR takedown and reassembly skills are all you need.
The polymer gas deflector redirects hot gas and powder residue that normally cycles back out the ejection port, keeping that thermal and particulate cloud away from your face. It's especially noticeable during high-round-count sessions and when shooting indoors.
The kit includes a steel ejection port cover (with detent hardware pre-installed), a polymer gas deflector, a roll pin, and a detent spring. Everything needed for a complete installation.
