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.

    Djcala and Scoop like this.
  2. Scoop

    Scoop 20g

    ^^^ 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 .223 Supporter

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

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