Saturday, August 28, 2010

AR Build - Lower Parts Install

After week-long boondoggle of UPS mishandling my package, my lower parts kit finally arrived today.

I ran up to Harbor Freight to get some punches after arriving home. Apparently they do not carry roll pin punches at the North Austin location. I opted for a simple multi-size set of steel punches in black oxide finish (they also don't carry anything but a honking 5/8" punch in brass), along with a nylon/rubber mallet.

I had a hard time coming into this build figuring out exactly what I needed in the way of tools. Certainly there are armorer's sets for building an AR, but those are prohibitively expensive for the first-time builder. Here is what I have learned are the essentials for putting a lower together:

A Bolt Catch Punch
I learned this the hard way today, by ruining my bolt catch roll pin. If you look closely at the photo (you can click it to enlarge), you may be able to see the mangled end of my magazine catch roll pin slightly protruding from the rear of where it's supposed to be. This will have to be replaced with my next order of parts. An ordinary punch WILL NOT fit there, because the shank is centered in the punch, and the handle can't fit against the lower, parallel to the pin. These punches can be found from pretty much anyone who sells AR tools. Both Midway and Brownells have them available. The trigger guard roll pin is also the same diameter, so this punch can be used there as well.

A Nylon mallet
I picked one up from Harbor freight for about five dollars, but you can find them at all kinds of places, including Midway and Brownells. This is useful for using your punches, but also helps you finish off installing the trigger and hammer pins without marring the finish of your receiver and pins.

Specifically, 1/16, 3/32, and 1/8 inch sizes. While the 1/16 is not strictly necessary, It's nice for lining things up while you're working. The 3/32 is good for the trigger guard release, and the 1/8 can be used to get the mag release completely screwed in to the button.

Now, these are the things I found that were needed for a successful install. Here are the things I wish I'd had today:

A Vise and a Lower Receiver Vise Block
While you can put together your lower without these, there's only so many fingers you can use while holding the lower in your hand. A vise will keep you from making mistakes due to the distraction of holding the receiver. It will also allow more precise work. You can get a vise at any hardware store/mega home improvement mart. Vise blocks appear to be available from pretty much anywhere that sells AR tools, including Midway and Brownells.

A Pivot Pin Installation Tool
Installing the pivot pin spring and detent can be a pain, and one slip can send your detent flying across the room, never to be found again. I was able to avoid this fate with some finagling, but a purpose-built tool for this would have certainly sped things up and lowered the risk. Again, this can be found at places that sell AR tools, including Midway and Brownells.

So there you have it. My lower is mostly assembled, The buttstock (M4-style telescoping carbine) is already on order, and should arrive in about a week. Then, I move on to the expensive stuff. Everything besides the CMMG stripped lower is being assembled with parts ordered from Rock River Arms.

Thanks to aepilotjim and the rest of the folks from the GBC for their assistance in this endeavor. For a good guide to assembling your AR, check out the How To Build An AR-15 videos over at Brownells.


  1. Actually, you don't really need any special tools (although it does help to have an extra set of hands or some sort of vice to hold the lower) to put in the bolt catch pin or the pivot pin.

    The method I used for the bolt catch pin was just to use a long pin punch. You can get a set of 5 long pin punches from Harbor Freight for $4.99. They always have them in stock in the store in this area, but even if they aren't in your store, you can order online.

    The other method to install the bolt catch pin, I've seen online but I've never tried. Basically, they use a pair of vice grips with the jaws padded with duct tape and squeeze the pin into place.

    As far as the pivot pin, Here's the method I used. Worked great and no special tools required.

    I also assembled an upper from parts for my National Match Rifle. I revisited the lower assembly in that post series, but probably not as thoroughly as I did the first time. I covered assembling the upper in my normal excruciating detail though.

    Here's the post series on that build if you're interested.

  2. Looking forward to seeing the rest of the rifle!

  3. Sailorcurt: Thanks for the commentary. I wish I would have known about your build documentation before. While the Midway videos are good, they occasionally are a bit thin on the visual orientation instructions. Your instructions are quite nice in that aspect.

    The vise grips (needle nose) method is actually what did in my catch release pin. I didn't realize that the lever wasn't quite lined up when putting it in, and the pin ended up a little bent and mangled on the end. My fault, really.

    As for the long punches, they definitely didn't have those at the Harbor Freight I went to, or I would have picked them up.

    One question about your build. How is that Sight-Mark red dot working out for you?

    JP: So am I!

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