As any avid AR-15 shooter knows buying ammo for your range trips can be a painful experience. A fast trigger finger will not only put holes in your target but also your budget. In a search for cheaper to shoot solutions, I stumbled across one of CMMG‘s newest conversion kits. The Resolute 300 Mk4 22LR AR-15 upper receiver kit.
CMMG has been making drop-in 22LR conversion kits for the AR-15 for years. The Resolute series stands out because it is a complete upper receiver boast. The 300 series Resolute upper I received has the following specs:
- CALIBER: 22LR
- BARREL: 17in, 1:16 twist, MT profile, 4140 CM, SBN
- MUZZLE: CMMG SV Brake, threaded 1/2-28
- GAS PORT LOCATION: N/A
- RECEIVER: Forged 7075-T6 AL M4 type upper. Complete with BCG
- HAND GUARD: CMMG RML15 M-Lok hand guard
- FINISH: Hard Coat Anodized Receiver and Hand Guard (Can be upgraded to Cerakote)
- CHARGING HANDLE: CMMG Oversized Ambi
- WEIGHT: 4lbs 1oz
- LENGTH: 25.3″
With a 17″ barrel and weighing in at just over four pounds, the resolute upper looks and feels like a full-size AR upper. CMMG also offers these uppers in various Cerakote colors and sent this one over in Sniper Grey.
Removal of the bolt carrier assembly is done the same way as a standard AR-15. Simply pull the over-sized charging handle rearward to expose the operating assembly, and then pull rearwards to remove it. Unlike a standard AR-15 Bolt Carrier Group (BCG), the Resolute upper uses a straight blowback design. So you could run this upper without a buffer tube assembly if you wanted to.
As there is no need for a gas tube, the 17″ barrel free floats underneath the slim MLOK handguard. The barrel has a 1:16″ twist rate for stabilizing both medium and high-velocity 22LR projectiles. To top things off the Resolute 300 series comes with CMMG’s SV muzzle brake. This uses a standard 1/2-28 thread pitch and can be easily swapped for your favorite suppressor or muzzle device.
CMMG was kind enough to send four of their 25 round magazines over for this review. These have the same look and feel as your standard AR magazines while remaining relatively easy to load.
At The Range
Various AR lower and trigger combinations were tried before settling on this Daniel Defense lower equipped with a Franklin Armory Binary Trigger. For testing, I decided to use the Vortex AMG UH-1 as it’s my go-to all-around AR optic.
Throughout testing, I used a mix of lead and copper jacketed bullets in various velocities and weights. The upper didn’t seem to favor a particular type of ammo (as 22LR guns often do). I would infrequently get light primer strikes with cheaper ammo, but nothing I wouldn’t expect from cheaper ammo. Sticking with a case of Federal bulk and CCI subsonic the upper ran almost flawlessly.
With the upper installed the gun feels and points just like a mid-length AR-15 would. Minor differences aside the CMMG upper is very close in terms of handling. Features like a true last round bolt hold open are missing, but after a few mag dumps these small missing features were quickly forgotten.
Where the upper really shines is with the addition of a silencer. By removing even more of the noise and recoil it’s very easy to focus on the basics when running drills.
Full Auto Use
Prior to the start of this review, I asked CMMG if their upper was full-auto rated (for science reasons). They quickly explained that it was, and sent over their Auto Sear Trip Kit with the upper receiver.
The auto sear trip kit is comprised of just two pieces that are added to the existing bolt.
The first is a heaver forward assist mechanism to replace the factory plastic one. The added weight behind the bolt aids in eliminating bolt bounce.
Secondly is the small bar that sits on the bottom of the BCG. This is to trip the auto sear in an M16 style full-auto assembly. And no, this won’t make your semi-auto gun full auto. You’ll still need a true to form M16 lower.
Mag after mag only saw a few failures and was an absolute blast to shoot. Oddly enough I did capture one of the few failures of that day on video. As you’ll see it was quickly remedied.
Pro’s & Con’s
I really like this separate upper compared to a drop in .22LR conversion kit. Throughout testing, it worked almost flawlessly using various lowers and ammunition. The barrel easily stabilized all the ammo I put through it and ran cleaner than I expected. Still, you can get it as dirty as you want and you’re just two pins away from swapping back to your regularly scheduled caliber.
As a perfect low-cost training solution the CMMG isn’t perfect. You’ll have to charge the weapon between each mag change, and with a fast trigger finger, you’ll be doing that frequently. The single stack 25 round magazines aren’t the easiest to load, and you’ll find yourself wanting more quickly.
If anything will stop you from buying this upper it will probably be the price tag. The Resolute 300 starts at $674.95, and the Cerakote adds another $100 to the price tag. There are other Resolute uppers from CMMG that start as low as $524.95, but these certainly aren’t the cheapest conversion kits on the market.
To those that enjoy a higher-end AR-pattern lower with all the trimmings, the CMMG Resolute upper is for you. You get to retain that higher-end trigger, grip, and stock assembly you come to love, all while shooting much cheaper ammo. While the startup cost may be high, I think it’s worth the money for a dedicated upper that just works.