Tag Archives: 80%

AR-10 / LR308 Lower Assembly Guide *No Video!*

With “Ghost Guns” coming up so much lately, I wanted to ride the wave and do something relevant to that discussion. I’ve also been looking for an excuse to put something together off of this 80% Arms .308 lower that I’ve had finished for a while. True to what this website was intended for, here’s a no-video step by step assembly guide.

I’m using an Aero Precision M5 .308 Lower Parts Kit, minus FCG/Pistol Grip, as I intend to supply my own trigger (and you should too). Also needed is a receiver extension / buffer assembly – I’m using an Aero M5 .308 Carbine Buffer Kit, which also includes the end plate and castle nut.

Tools needed:
Screwdriver – whatever kind you need for your grip screw, varies.
Castle Nut Wrench – only if using a carbine receiver extension.
1/16th Punch – Not necessary, but definitely helped a few times.
7/16th Roll Pin Punch – Same as above, not needed but helped.

We’ll start with the Magazine Catch.
You’ll need the Magazine Catch Body, Spring, and Button.

Drop the Body into the recess on the left side of the lower.

Then put the spring to recess on the opposite side, and push the button in on top of it.

Here’s first fun part. You’ll want to push the button in while holding the body on the other side, and push the body out slightly while holding the button. This should push the body out of the recess. Rotate it a few times clockwise (about 3 turns) so that it’s started and you don’t have to worry about losing it.

Now, use something that wont mar the finish to push the button further in that you can by hand – a dowel or plastic pen work. I used a 7/32 roll pin punch to do it. With the button pushed in as far as it’ll go, continue screwing the body in until another rotation would be scraping the receiver (don’t do this), and let go. Done.

Next, the Bolt Catch.
We’ll be using the Bolt Catch, Buffer, Spring, and Roll Pin.
Note: LR308s, including this 80% Arms lower, generally use a threaded Bolt Catch pin. The Aero kit doesn’t include this, and honestly a roll pin works fine here, just as it does on an Armalite AR-10 or an AR-15. But if you’d rather use a screw, you can buy one here.

Put the buffer into the spring, and drop that spring-side-down into the hole here.

Put the magazine catch into the recess, and simply push the roll pin in, wiggling things as needed until it goes all the way through. Use a small punch to push it in just a tad deeper than you can with your finger, and that’s it. Tension from the spring will keep it in place.
Note: If you’re using a screw instead of a roll pin, simply thread it in using the proper size hex wrench.

Now, the hard part – Pivot Pin Assembly.
We’ll be using the Pivot Pin, Pivot Detent, and Pivot Spring.
Note: The detent and spring for the Pivot and Taketown pins are interchangable.

See that little hole in the front there? Put the spring in through there. It’ll go right into a recess in the receiver behind it. Now, the trick here if there is one, is to put the spring in partially, then follow it with the detent, then slide the whole in through in one motion by following it with a 1/16″ punch or similar. Try your best to do this in under 30 attempts.

Isn’t it great that those detents are brass, and thus easy to find when you drop ’em the 7th @#$%ing time?

Now, with it held in place (it’s okay, I know that took a few tries), start the pivot pin in with the groove facing the detent. When you’ve got it started and it’s pushing against your punch, in as smooth a motion as you can manage, remove the punch and push the pin in. With luck, you’ll get a satisfying *click* and can then push it through the rest of the way with another *click* when it seats all the way through.

That was the hardest part, honest. It’s all downhill from here.

The next bit includes a few things done at the same time.
Takedown Pin Assembly, Safety Selector, and Grip.

Important note – as mentioned at the start, I’m using a trigger other than the one included in a standard lower parts kit, and wont be covering it’s installation in this guide. It is at this point in the process that you will install your trigger. Proceeding past this point without installing a trigger will require you to remove the safety in order to perform the installation.

For instructions on install of my preferred trigger, see this post, and start at step 7. Instructions are the same for Timney (and most other) drop in triggers on both AR-15 and AR-10/LR-308 platforms.

Okay, back to business –

First get the above parts ready – The Takedown Pin, Pivot, and Detent, and the Safety Spring and Detent.

Put the Safety Selector in, and set it to Safe. Then, put the Takedown Pin into this hole, groove side down (relative to the receiver). Next, put the detents into these holes, pointy side towards the parts you just installed.

Now, get your grip and screw ready, this will take a touch of finesse to not bend the springs, but isn’t too difficult. Put the Takedown Pin spring into the hole in the receiver, but put the Safety spring into the hole in the grip. Slide your grip on, while making sure the Safety Spring lines up, and push it home being careful to compress, but not bend, the Takedown spring.

Now, tighten the grip screw. In my case it was a hex head.

Almost Finished!

Finally we’ll be installing the Receiver Extension & Buffer Assembly.
I’m using a carbine buffer kit, which has a few extra steps. If you’re using an A2 length extension, you can mostly ignore the bits about the End Plate and Castle Nut.

First, thread the Castle Nut onto the receiver extension with the deeper notches facing the rear, like so. It’s okay if you run it all the way down and off of the threads.

Then, slip the End Plate on, dimple facing this way, down onto the extension by lining the notch up with the grove in the threads.

Now, place the Buffer Retainer Spring, and Buffer Retainer into the hole in the threaded area of the receiver.

Thread the receiver extension into the receiver until it touches the Buffer retainer, then back it off just as much as is needed to have it oriented correctly.

Slide the End Plate down until it mates with the receiver. The notch+cutout and dimple+recess will ensure the extension is properly indexed. Now, simply thread the Castle Nut back in the other direction until it’s tight.

You’ll want to use a castle nut wrench to torque this down to about 40lbs, then stake it using the forward facing notch if you’re so inclined.

Finally, run the spring and and buffer into the extension, and it should click in place over the retainer. Install your desired stock (or brace) if you like, and you’re done with the basics.

Note that I didn’t include the trigger install. That’ll be along with a review coming up shortly – but if you know how to install an AR-15 trigger, it’s pretty much the same. In the meantime, see my review of the Timney Targa drop-in trigger for the basics on installing a drop-in trigger to an AR, it would apply here.

“My lower parts kit came with a very tiny screw, where does that go?”
That’s a 4-40 set screw for the takedown pin detent hole. On fancier receivers, that hole is threaded and you can compress the spring and thread that in with a hex screw instead of pushing it in with the grip.