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MMR Hunter and CMMG .22LR Conversion Kit

Discussion in 'Mossberg MMR Hunter And Tactical AR15' started by Nutt, May 22, 2017.

  1. Nutt

    Nutt Copper BB

    I started this thread to document experiences using the CMMG .22LR Conversion kit with the MMR Hunter with the hope that it may be useful to others.

    First things first: the CMMG .22LR Conversion Kit comprises a .22LR bolt carrier and a magazine. If you're patient and willing to shop around you can find these kits for around $160 + shipping. The mags come in two flavors: 25 round and 10 round. The 10 round magazine is just a plugged 25 round magazine (more about this later). I purchased the kit with the 10 round magazine.

    To use the kit you simply remove your .223 Remington/5.56 NATO bolt carrier and insert the CMMG .22LR bolt carrier. Then load the CMMG .22LR magazine, insert your magazine into your mag well, charge, and fire away. When the magazine is empty, the last round bolt hold open feature (LRBHO) will hold the bolt open until the magazine is removed. This is how things are supposed to work. But that's not what I experienced.

    On my first attempt I gently released the hammer, inserted the CMMG bolt carrier, closed up and attempted to cycle the bolt. On the return-to-battery half of the cycle, the bolt became bound. Luckily, since the .22LR bolt carrier has its own recoil spring (the bolt carrier does not travel into the buffer tube), I was able to remove the upper, then remove the CMMG bolt carrier with no apparent damage to either the firearm or the bolt carrier.

    After a quick email exchange with the good folks at CMMG, I quickly recognized I was experiencing a common problem: the kit may malfunction when using a “notched hammer.” From CMMG support:

    The conversion kit does not function properly with a notched hammer. It's performance can be sporadic, at best, or not at all. It's not a safety issue, really. The issue of burst fire is actually more due to a light trigger pull.

    The conversion kit requires only two things, really, from your fire control group: no notched hammer, and at least a 4 lb. trigger pull.

    A standard, Mil-Spec trigger group is your best bet for functionality with the conversion kit.​

    That sounds simple enough. But what's a “notched” hammer?

    What's a “Notched” Hammer Anyway?

    A quick inspection of the MMR Hunter's hammer answered my question. The photo below shows both a “notched” and a standard (or what some folks refer to as a “rounded” hammer when discussing the CMMG Conversion Kit).


    The next question I had was, “What's the purpose of a notched hammer?” Based on my research there is some disagreement among the rank and file on the firearm forums. The reason that sounded most plausible to me was that it provided some protection against slam firing if the fire control group has a faulty/damaged disconnector. The explanation goes something like this: if the disconnector is damaged/faulty, the notch in the hammer will bind with the firing pin and prevent the bolt from returning to battery.

    The actual purpose of the notched hammer really doesn't matter for the task at hand, but it did motivate me to play around with the CMMG bolt and the hammer – to see if I could determine what was happening. The two photos below illustrate what I believe to be the issue when using the CMMG 22LR bolt carrier with the stock MMR Hunter notched hammer. Note that these photos were taken with the hammer and CMMG bolt carrier already removed from the firearm ... and arranging them on the bench top.

    A Close-up of the Problem

    A Wider View of the Problem

    So I replaced my hammer. This wasn't a big deal. For about $20 and about 10 minutes of work I had a new “rounded” hammer installed. If I knew what I was doing the entire replacement would probably have taken less than 4 - 5 minutes. Once the hammer change was complete, the CMMG bolt carrier could easily be inserted and removed from the upper receiver.

    Side Effects of the New Hammer
    If you carefully compare the two hammers, you'll notice a several other differences other than intended “notched” v. “rounded.” Whether or not these differences will create any long term problems, I can't say. Perhaps someone from Mossberg can comment on this.

    What I can say is that the new hammer appears to have introduced some additional trigger creep. Since I'm just so-so with a rifle, I doubt that it will affect my shooting significantly for the time being.

    Function Testing
    Initial function testing at the range was positive. I took me less than a minute to switch BCGs. The new hammer (other than the additional creep) appeared to operate just fine with the .223/5.56 BCG and did not bind when using the CMMG BCG. With the CMMG .22LR Conversion Kit installed I fired two strings: 5 rounds and 9 rounds (CCI 36 gr. Mini-Mag 22 LR HP) without any feed, fire, or extraction issues.

    The last round bolt hold open operated as expected. Since the bolt is basically held open by the magazine, it will close when the magazine is removed. I did have problems loading the 10 round magazine to capacity. The 10th round was just too tight of a fit, so I decided to use just 9 rounds for the initial testing.

    Anyway, there you have it. Hopefully this post will save folks some time and confusion if they're interested in the CMMG .22LR Conversion Kit. So far it seems to work just fine.

    I'll follow up with some performance reports after I get more time at the range.

    Mark_Johns, Djcala and Scoop like this.
  2. Scoop

    Scoop .30-06

    ^^^ Very nicely done, Nutt. Thanx for taking the time to prepare the research and share it with us. I'm interested in the follow-up.
  3. Djcala

    Djcala .30-06 Supporter

    Well done. i will also be interested and follow your progress.
  4. Mark_Johns

    Mark_Johns Copper BB

    Thanks for the info. I recently ran into the same problem. I'm ordering a new hammer for one of our MMR's and a Davidson Defense drop in trigger system for the other.

    Sent from my Moto G (5) Plus using Tapatalk
  5. Nutt

    Nutt Copper BB

    Update/Range Report

    I put about 500 rounds through the conversion kit to date ... and figured it was time for an update/report.

    General Impressions
    • Reliable so far. Not a single malfunction - no failures to feed, nor failures to eject, no jams. And I still haven't cleaned it - not even once. So I'm happy with the overall reliability.
    • Not bench-worthy - at least not if you're expecting nice, tight groups at 50+ yards.
    • Excellent for training - If you shoot handguns you probably know the value of dry-fire drills. IMO this kit lets you achieve the same goals, but live-fire. I've been able to eliminate flinch (when it shows up), avoid grabbing, etc. with cheap ammo - you can get in plenty of repetitions (without going broke) and get real feedback.
    • Great for off-hand practice - once you're off the bench, this kit is great. Again, lots of reps, little $$$.
    • Potentially competition worthy - I haven't used the kit for steel challenge/bowling pin matches yet. However, it's on the to-do list. It has proven reliable enough. And with targets at 7 -15 yards, it's accurate enough. My only concern is the weight of the MMR Hunter - which has nothing to do with the conversion kit.
    Range Results

    I follow a fairly regular routine:
    • With a clean barrel, three fouling rounds using .233 ammo.
    • Switch bolt carriers & shoot three to five 5-shot groups with .22LR.
    • Switch bolt carriers again & shoot 3-shot groups with .223 until I'm content or run out of ammo.
    In order to take anything away from the photos (below), you'll have to consider the equipment behind the butt stock. Below is a typical set of 3-shot groups with the .223 ammo. Group 1 is the fouling group and Group 2/Group 3 is after switching back (from .22LR to .223). Disclaimer: I'm not a particularly good rifleman. To keep things reasonable, all of the photos are targets shot at 50 yards.


    Now that you have something you to judge my shooting, you can judge the CMMG performance for yourselves.

    CCI Mini-Mag, 36 gn

    This is a typical 5-shot group with the 36 gn CCI Mini-Mag - including the shot at the bottom-left. I rarely get all five shots in a clean cluster.


    CCI Velocitor, 40 gn

    And this is a typical 5-shot group using the 40 gn CCI Velocitor. This round is more expensive but (typically) performs a bit better than the Mini-Mag (at least for me). And as before, I struggle to keep all five shots in a nice cluster.


    Anyway, there's some actual targets using the conversion kit in the MMR Hunter. I'll post a few more photos if/when I get a chance to try some different ammo. The next on the wish-list is the CCI subsonic.

    Djcala likes this.
  6. Nutt

    Nutt Copper BB


    In my earlier post I mentioned the additional creep with the new hammer. At first, it didn't bother me much. Now I find it annoying, at best. I find myself wondering, "Man, when's this shot finally going break!" -- and a respectable group turns into a disappointing group all too often.

    Please let me know how that drop-in trigger works out for you. I'm sure I'll finally have my fill and start looking for something better.

  7. John A.

    John A. I'm "THAT" guy Staff Member Global Moderator Moderator

    Without getting too technical, there is a way to reduce creep in a factory trigger.

    You do so by removing as much of the metal as you need to on the contact face of the hammer itself.

    But by doing so, you take the chance of the hammer slipping off of the face of the trigger and firing inadvertently when you are not intending to. ie., drop firing.

    The main reason why the 22lr is not as accurate as .223 is the .224 barrel wasn't never designed to shoot 22lr. A long jump in the chamber with the conversion bolt/chamber insert, and a larger bore itself, are both working against the 22lr ever seeing "Olympic grade" accuracy.

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