AR-180 Magazines: Modifying AR-15/M16 M4 magazines to fit an AR-180

Designed by Eugene Stoner and Arthur Miller, the Armalite AR-180 was produced by a variety of different manufacturers between 1969 and 1985. The rifles aren’t particularly common in the United States and original magazines are expensive when you can find them. One of our customers is lucky enough to own a Howa AR-180 and wanted magazines that he could shoot with the rifle. He asked us to modify a bunch of Mil-Spec AR-15/M16 M4 magazines to work with his rifle.

Since the AR-180 (and its select fire cousin, the AR-18) are predecessors of the AR-15, you’ll notice some strong similarities in the external dimensions of the detachable box magazines. While they are largely similar, modification of the magazine to work with an AR-180 requires the removal of the magazine stop bump from the left side of the magazine, lowering the spine of the magazine, new magazine release slot cut and a notch cut in the rear of the follower.

I took a number of picture of Howa and Sterling produced OEM magazines next to the Mil-Spec magazine for comparison purposes. Additionally, I found this old drawing that lists some dimensions that can be used as a starting point to fit the magazines on your rifle.

This first image shows, from left to right, the right side of a standard 30-round AR-15 magazine, a 40-round Sterling AR180 magazine, and a 20-round Howa AR180 magazine. Note the fairly narrow slot for the magazine catch. The placement of this cut is critical for the magazine to properly function.

The left side of the AR180 magazine does not require a cut, even though the Sterling manufactured magazine (center) has one. The stop bump on the AR-15 magazine needs to be removed to work with an AR180.

While it is somewhat difficult to make out in this picture, the top of the rear “spine” of the AR-15 magazine needs to be slightly lowered to work in an AR-180.

I managed to find the image above on the internet. These dimensions worked for the Howa manufactured rifle we had in the shop.

Originally I had planned on working these magazine over in my CNC milling machine. The more I thought about it, the better the idea of machining these on a manual machine seemed. Using a stop and series of parallels, I’d be able to position each magazine in the same place for each operation. I wouldn’t have to worry about soaking the parts in coolant, and since I was simply cutting thin aluminum, a manual machine would have ample power for the task.

This task didn’t require a lot of specialized tooling beyond a 3-flute 1/16″ end mill to cut the magazine catch slot. Since the operation requires a plunge cut to a slot, I ordered a few extra in case I broke some (I ended up only breaking one). Other than that, I made the rest of the cuts with a 2-flute 3/8″ end mill.

After examining the magazine and thinking about how it interacted with the rifle. I decided to remove the magazine stop, then notch the rear of the magazine and finally cut the magazine catch slot. I wanted the ability to adjust the slot as needed to fit the rifle and if I made it first, I wouldn’t be able to do so.

To remove the magazine stop bump, I set up a work stop and a set of wide parallels in the milling machine. I used a 3/8″ 2-flute end mill to cut it out.

I simply made a plunge cut over the AR-15 magazine release hole and moved the magazine .380″ in the x-axis. This cut out the entire magazine stop.

Next, I cut the notch on the spine of the magazine. To set this up I used a work stop to cut the notch down .570″ from the top edge of the magazine feed lips. Since the factory magazine also has some protrusion in the notch, I made sure this and the new cut were at the same depth for a more finished look.

Of all the operations, the magazine release cut is the most critical. Too high and the magazine will not seat, too low and it will not feed. The print shown earlier shows the notch centered 1.341″ down from the top of the magazine and starting 1.51″ from its front edge. The slot is .48″ long. I decided to use a 1/16″ 3-flute end mill to cut this slot. After measuring a few magazines on hand, I decided that .0625″ diameter of the end mill, was best for the task. Additionally, run out in the mill or a small file after the operation would open the magazine slightly if needed. In terms of common tool sizes, a 2mm end mill would have made the slot bigger than the drawing (.079″), so 1/16″ seemed like the way to go.

After the magazine body was cut, I still needed to modify the followers. To do this I set the follower on a thin parallel in the milling machine vise. Once I set the x and z-axis for the cut, I simply made the cut by moving the y-axis.

A thin file made short work of deburring the magazine catch groove.

Success! This AR-180 owner now has 9 new magazine he can use at the range!

If you need to convert AR-180 magazines, show this post to your local gunsmith, he should be able to help you. If not, you can contact us at