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.
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.
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.