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The CMMG Banshee 5.7x28mm upper is about as compact as an AR-pattern build gets. That short 5-inch barrel makes this the smallest 5.7 upper group in the CMMG lineup, so if you're chasing a tight, handy PDW-style setup, this is where it starts. The 5.7x28mm round has earned its spot as a go-to personal defense caliber thanks to flat-shooting ballistics and light recoil, and it punches well above what you'd expect at closer ranges while still delivering solid penetration.
The real magic here is CMMG's Radial Delayed Blowback system. Instead of relying on a heavy bolt and buffer like a straight blowback design, the rotating bolt bleeds off a portion of the rearward energy as it unlocks. That lets CMMG run a lighter carrier, which means a softer recoil impulse and faster, easier follow-up shots. It's the same operating principle that lets the AR platform handle calibers it normally struggles with, and paired with the ZEROED linear compensator, the whole package stays smooth and controllable.
The upper receiver is machined from billet 6061 T6 aluminum and wears an Armor Black Cerakote finish, wrapped up with a free-float CMMG EML4 M-LOK handguard for mounting lights, grips, and whatever else you run. The barrel is 4140CM chrome-moly with an SBN finish, cut with a 1:9 twist and a 1/2×28 threaded muzzle, so swapping the comp for another muzzle device or a can is straightforward down the road.
One thing to know before you order: this upper feeds from 5.7 pattern magazines, not standard AR15 mags. It drops onto CMMG Mk57 lowers directly, or onto a standard AR15/Mk4 lower when you're running CMMG's 5.7 AR Conversion Magazines. It ships as a complete group with the bolt carrier, ambi charging handle, dust cover, and linear comp already installed, so you're basically ready to mate it to a lower and head to the range.
Yes, it fits a standard AR15/Mk4 lower, but only when you use CMMG's 5.7 AR Conversion Magazines. It also mounts directly to CMMG Mk57 lowers, which run FN Five-seveN pattern magazines. Keep in mind it will not feed from standard AR15 magazines.
The rotating bolt uses part of the firing energy to unlock before the carrier travels rearward, which lets CMMG run a lighter bolt and buffer than a straight blowback design. The result is a noticeably softer recoil impulse and quicker follow-up shots. It also helps the AR platform run a round like 5.7x28mm reliably.
It ships as a complete upper group with the bolt carrier, ambi charging handle, dust cover, and linear compensator already installed. Once you pair it with a compatible lower and the correct magazines, you're set. No gas system to assemble since the Radial Delayed Blowback design skips gas tubes and blocks entirely.
The barrel is threaded 1/2×28, so mounting a suppressor or a different muzzle device is simple. CMMG also offers an Action Tuning Kit with carrier weights if you want to dial in reliability when running suppressed or with hotter loads. Many owners appreciate how the 5.7 round performs velocity-wise behind a can.
| UPC | 810097504455 |
|---|---|
| Manufacturer | |
| Manufacturer Part Number | 57BBCF0AB |
| Action | |
| Barrel Length | 5" |
| Caliber/Gauge | 5.7 x 28 mm |
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