Before we get started, I want to correct a misnomer that I’ve fallen into and I hear pretty commonly.
On a Carbine AR this is not the buffer tube, it’s the receiver extension and there are two sizes available: the commercial version, which is 1.17 inches in diameter, or the mil-spec version, which is 1.14 inches in diameter common on Colt rifles.
If you’re simply going to be changing your adjustable stock on your carbine, a set of calipers will help you determine which one you’ve got.
Three-tenths doesn’t sound like a lot, but it will bite you in the butt, if you don’t know.
For our project, we’re going to be converting a rifle style AR into a carbine. We’ve selected a mil-spec receiver extension and a Magpul CTR Adjustable Stock, as well as a new spring and buffer.
The first step in the process is to check for a clear rifle, then separate the upper and lower receivers by popping the pins.
Now, I need to secure the lower receiver so I can work on it, using the Brownells Lower Receiver Vise Block.
This detent here will interface with the magazine catch inside the lower receiver so I know it’s secure. Then I want to chuck it up in the vise at a slight down angle to help me retain the takedown pin detent and spring.
Next, I’m going to use the large flat head to remove the buttplate. With the stock off, I’ll press down on the buffer retainer and then remove the buffer and the spring.
It’s now time to remove this receiver extension. To do that I’m going to use the Brownells AR Carbine Wrench and this slips right over the end. Keep in mind, the buffer retainer has a spring under it, so you want to keep your finger on it as you loosen the receiver extension, so it doesn’t go flying across the room.
For the new receiver extension, I have the castle nut and the receiver end plate all the way down, and it’s a little tricky, because the receiver end plate rides in the notch, but it doesn’t turn.
So, I need to thread it in far enough that the front lip holds buffer retainer in place, but doesn’t disturb the takedown pin detent and spring. We’ll start by threading it in, and there it is.
Then, we’ll slide the receiver end plate forward and tighten the castle nut by hand.
This is when the Brownell’s AR Carbine Wrench really shines. It has four teeth, while other wrenches on the market have only three.
So, align it with the castle nut and tighten it down, then do the same with the carbine length buffer and spring.
Lastly, for the Magpul buttstock, slip it into place, and pull the adjustment pin all the way. Finally, we just pin the rifle back together, and that’s it.
