Lapping AR15 Uppers

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  • omegared24

    Ultimate Member
    Dec 23, 2011
    4,747
    Ijamsville, MD
    I'd like to thank DaemonAssassin for his generosity. He set aside time and resources to teach me how to lap an upper. He really knows his stuff.

    I brought 2 BCM blems ($59 each...they come with forward assist and ejection port covers). I also brought 2 garbage blem uppers that I purchased a while back. They were super cheap at $29. AO Precision if I recall correctly. Not to be confused with Aero Precision.

    Interesting lesson about quality. You could tell very quickly how much better the BCM product is compared to the cheaper option. It took no time at all to remove the anodizing from the AO upper. The BCM took a good bit of work to properly remove the anodizing.

    We had to be really careful inserting the lapping tool in the BCM upper as it was a tight fit. The AO upper wasn't as finicky.

    Here are pics of each brand after lapping. You'll notice the BCM upper still has a little anodizing on it.
     

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    clandestine

    AR-15 Savant
    Oct 13, 2008
    37,031
    Elkton, MD
    Good work.

    For those reading here is an additional Tech Tip:

    Some uppers require you to heat the front portion to install a barrel. This is called thermal fit. The first 1" of the upper is tighter than the rest to achieve this function.

    Thermal fit uppers usually won't let the lapping tool go into the upper receiver without heating it up.

    On a thermal fit upper, you should never heat just the front in order to lap it, you should evenly heat the entire upper (oven @200 degrees). If you lap with only the front of the receiver warmed up it can cause an uneven lap due to heat variations.
     

    omegared24

    Ultimate Member
    Dec 23, 2011
    4,747
    Ijamsville, MD
    I’m curious to see grouping before and after lapping. I’ve always wondered how effective it is.

    DaemonAssassin has a great example of one he did. From what he was telling me it made a huge difference.

    If I ever find the time I may go ahead and lap one of my "beater" uppers and do a before and after test.

    Good work.

    For those reading here is an additional Tech Tip:

    Some uppers require you to heat the front portion to install a barrel. This is called thermal fit. The first 1" of the upper is tighter than the rest to achieve this function.

    Thermal fit uppers usually won't let the lapping tool go into the upper receiver without heating it up.

    On a thermal fit upper, you should never heat just the front in order to lap it, you should evenly heat the entire upper (oven @200 degrees). If you lap with only the front of the receiver warmed up it can cause an uneven lap due to heat variations.

    I was worried that we would have a problem because I've heard stories of BCM uppers being so tight but it wasn't an issue here. Thanks for adding this info as I would of assumed you just do the front.
     

    Qbeam

    Ultimate Member
    Apr 16, 2008
    6,074
    Georgia
    I know this is going to sound like a stupid question, does the lapping affect the headspacing of the barrel?

    I would think not due to the bolt "floating" somewhat in the bolt carrier to allow lockup with the barrel.


    Q
     

    clandestine

    AR-15 Savant
    Oct 13, 2008
    37,031
    Elkton, MD
    I know this is going to sound like a stupid question, does the lapping affect the headspacing of the barrel?

    I would think not due to the bolt "floating" somewhat in the bolt carrier to allow lockup with the barrel.


    Q

    Headspace is not effected by lapping. It can however cause M4 ramps to overlap and cause feed issues.
     

    DutchV

    Ultimate Member
    Jul 8, 2012
    4,703
    <snip>

    I was worried that we would have a problem because I've heard stories of BCM uppers being so tight but it wasn't an issue here. Thanks for adding this info as I would of assumed you just do the front.

    BCM hasn't always made their uppers that tight. For awhile I believe the tight one was a separate part number. I know I have an older one of theirs that is not overly tight.
     

    outrider58

    Eats Bacon Raw
    MDS Supporter
    Jul 29, 2014
    49,818
    Ooooh, you said that special word, hold my beer.....


    Q:D

    Let me clarify. To do this(cutting and polishing M4 type feed ramps), the barrel must be removed for this operation. You never want to risk cutting into the feed ramps of the receiver itself. It is done slowly, checking multiple times as you go, for a proper fit.
     

    Qbeam

    Ultimate Member
    Apr 16, 2008
    6,074
    Georgia
    Let me clarify. To do this(cutting and polishing M4 type feed ramps), the barrel must be removed for this operation. You never want to risk cutting into the feed ramps of the receiver itself. It is done slowly, checking multiple times as you go, for a proper fit.


    Roger, just having some fun.:thumbsup::D Definitely a process that requires focus and not rushing to finish.


    Q
     

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