AR-MPR: Build Part 4 – Upper – Bolt Carrier Group and Headspace

In this video, we’ll put the final touches on the upper assembly by walking through the assembly of the Bolt Carrier Group (BCG), checking headspace, and finally installing the BCG and charging handle into the upper receiver.

Note: Here, I show you how to check headspace with the extractor installed. You can also do this check without the extractor installed and have better “feel” for the headspace measurement/check. Note that you’ll have to tap out the headspace gage with a cleaning rod if you go that route.

Next, it’s on to the lower assembly!

9 thoughts on “AR-MPR: Build Part 4 – Upper – Bolt Carrier Group and Headspace”

    1. Norton- if you have a headspace problem, it’s time for some gunsmithing! Either the bolt is not in spec, or the barrel/chamber needs work.

  1. Gavin:
    Question, If all the BCG are built to SAMMI specs and all the AR parts are interchangeable then why would you need to check the headspace on a new assembly? I recently traded out a new BCG for a new chrome BCG on a new rifle and I also thought that headspace would have to be checked. After talking to a colt certified armorer and gunsmith he told me it was not necessary on a new rifle. Any thoughts on this? Made sense to me.

    1. You really shouldn’t have to check if you assume all parts are within spec, but if you’re buying parts from several manufacturers, it’s a “better safe than sorry” kind of thing.

  2. I believe you may already know this but I wanted to make a note about head space gauges. There are three sizes of head space gauges available.

    go gauge- Made to the minimum head space specification, the bolt should go fully into battery with this gauge in. If it does not the chamber is too tight.

    no go gauge- Made to the maximum head space specification, the bolt should not go into battery with this gauge in. If it does the chamber is over size.

    field length gauge- Made over the maximum head space specification. If the bolt goes into battery with this gauge the chamber is unsafe.

    As always your videos show great camera work, lighting, and commentary. I look forward to watching more of them and seeing your new AR shoot tight groups.

  3. Would love to have more info on checking the headspace. I have googled over this topic many times, got some basic understanding, but it’s nice to see someone actually go through the steps in a video. I still have a few questions though.

    1) Which gauge were you using? (Go?) I see that Brownell’s sells a regular .223/556 gauge and a “match’ gauges. No detail on the differences on their website.

    2) When you are describing the “feel” in your note to the video, what exactly are you after here. Should it be tight, but not too tight, etc? Was the primary part of the shove you did on the back of the bolt just to get the extractor over the rim? Please elaborate a bit. I would think without the extractor would be the way to go, even if you have to use a rod to get the gauge out.

    3) If I’m understanding the differences in the go and no go gauges, wouldn’t it be a good idea to use both when re-barreling? (A go gauge to check minimum, and a no go to make sure it doesn’t exceed maximum?)

    Not questioning your methods, just trying to understand. Thanks for all the great videos Gavin.

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