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M261 Conversion Kit

At one time the US Military used conversion kits for training with the M16 rifle at reduced range. A cost saving measure no doubt. My brother was in the Army National Guard in the 80s and told me many times of them shooting their M16A1s at reduced range targets using the M261 rimfire conversion kit.


You will still see these turn up for sell online and gun shows etc. Now a days there are a variety of conversion kits on the market and just as many that have come and gone. CMMG makes one, Spikes made one, Colt even made one back in the day. The M261 was the one of the main adapters used by the military though.

Using it is as simple as it gets. You take your BCG out of your Ar15 and put this one in. Even AOC could manage it with a little help.

The magazines consisted of 10 round inserts that went into the standard USGI magazine. You just pushed them down inside like you would if you were loading 556 rounds.

pretty simple but at the same time you had to get them in just right to work

I would show you pictures of the inserts but I don’t have access to them. Instead the kit was used with the after market Black Dog Machine magazine. It works so so, has a bolt hold open and offers more ammo than 10 rounds. I have had less than great luck with BDM mags with a colt conversion kit but I won’t recommend them to you.

The adapter goes into the upper and into the chamber of the gun. The position that goes into the chamber is its own .22 long rifle chamber. It of course then uses the .22 caliber rifle barrel to fire 22 caliber bullets.

Once in the gun, you can close the dust cover and everything functions more or less like it would normally.

The kit as it comes for military use did not have a bolt hold open. The black dog machine mag does though to release the bolt the mag must be removed, It doesn’t function like normal. But if you want a hold open you got it. In some form anyway.

So how does it shoot? Not too bad. You won’t have very good reliability with standard velocity ammo. The twist rate will matter. 1/7 twist is a bit fast for 22 rimfire and the chamber in a chamber isn’t exactly match. it is great for plinking though and if you put this in an older 1/12 twist barrel you will get better accuracy. I have found the M261 needs a very light coat of oil. The ones I have used over the years need oil to work reliably. Too much and it won’t work. Too little and it won’t work. It’s a weird Goldilocks zone. Also, high velocity ammo is a must i you want it to work every time.

I fired this one in an M16A2 Colt upper at 50 yards and 100 yards using the iron sights off a rest. I did not bother to “zero” the gun for the kit so its a little off, but you can see the accuracy potential. The non-stop thunderstorms this summer ended further testing. But we will revisit this later this week.

two groups shot at 50 yards
100 yard group

I don’t recommend you buy one of these kits. They are finicky and a bit of a PITA. They are neat little bits of cold war history but thats about it. There are better kits and the S&W MP22 is faaaaaaaaar better if you want to shoot 22LR in an AR and they have real bolt hold opens and function identical to a center fire AR15 as far as fire controls , manual of arms etc. And they have proper 22LR barrels, chambers and twist rates. We will be comparing a couple kits to a MP22 later this week to see how much better it is.

11 thoughts on “M261 Conversion Kit”

  1. Very cool.
    I messed around with a ceiner.22 conversion, but to get more accuracy, I bought a complete CMMG .22 upper.
    Combined with one of my lowers, M&P 1522 magazines and a better mag adapter, ( https://www.brownells.com/rifle-parts/receiver-parts/caliber-conversion-kits/ar-15-m16-better-mag-s-w-m-p-15-22-magazine-conversion-adapter-prod56474.aspx )
    It is very accurate and has a last round hold open that functions like a 5.56.
    Also allows me to use the two stage trigger in my existing lower.

  2. The Air Force had a specific sub-cal adapter they used in the late 70s-early 80s. They were still technically in the inventory when I came in but never heard of them being used. Some units used the SRTA bolts and ammo though for SDZ/range limitation reasons.
    I had a 261 kit some time back. There were a couple shops that’d turn a custom barrel for them so you could ditch the chamber adapter. Mine was finicky too but after I took some crocus cloth and polished down the bearing surfaces a little and cleaned the decades of gunk out of the innards it worked maybe 85-90%. Good enough for plinking like you said.

    Didn’t Colt used to include a .22 adapter with new AR15s up to about the same time.

  3. 1-in-7 isn’t a “bit fast” for .22LR. It’s way, way fast.

    Normal twist for a .22LR, standard velocity round is 1 in 16.

      • Who’s complaining? I’m just stating a fact.

        Trying to spin up a lead bullet in rifling with that tight a twist will probably result in gas blow-by. It was for these types of reasons that guys like HM Pope experimented for so long with gain twist rifling in match barrels for shooting lead bullets.

          • I’m not taking offense or anything – I just put that fact out there because in my (admittedly limited) experience, there’s lots of people who know what twist is in their ’06 hunting rifle, their .223 varmint rifle and (especially) their AR-15 – but when you ask them “what’s the twist for a .22LR?” or “what’s the bore diameter of the .22 rimfire?” you get lots of blank looks.

            The reason why, I think, is that most all of us never, ever had any choice in the matter. Most all .22LR rifles (and pistols) we’ve purchased had whatever twist was in the barrel, and we never, ever, ever had to give it another thought, because we never, ever had to replace the barrel on a .22LR. Whatever it came with worked, and that was that.

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