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Thread: Mac 10 Upper Receiver Differences

  1. #1
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    Mac 10 Upper Receiver Differences

    Can anyone tell me if there are differences in Mac-10/45 upper receivers. I recently bought a (supposedly) vintage chrome Mac 10/45 upper and found that it didn't fit my RPB lower. Upon examination and comparison with my other upper it seems that the front end of the upper needs to be machined more in order for the upper to fit. Also, the receiver pin hole appear larger. Other than that the sizes appear identical. Is that normal? Does anyone know what kind of a Mac 10 is this chrome upper for? If I were to machine that part off is it all good to go? I was hoping to chrome my lower and get this to fit.

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  2. #2
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    RPB beefed up the lower receiver with metal tabs to keep the receiver pin from egging out the holes in the sheet metal. I see no reason why you shouldn't be able to mill your upper to fit. This is a common issue with people that are excited to buy an original Powder Springs upper and soon discover that it won't fit a newer lower...especially one of the semi open bolt pistol versions.
    Whatever you do, do NOT modify the lower even though that might be the obvious easy quick fix.
    "All that is necessary for evil to triumph, is for good men to do NOTHING!"
    Edmund Burke 1729-1797

  3. #3
    Registered User timkel's Avatar
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    The chrome upper is vintage(early) RPB (pre- reinforcement tabs).
    Pic- Top 3 uppers are Powder Springs. Bottom upper is early RPB.

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    Thanks, I figured there were some here that knew. Also, why are there 2 holes in the front on the vintage upper, and the receiver pin hole seems larger, is it ok if I made my lower pin hole bigger?

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    Do not modify the lower. Install a bushing in the upper if you must use it.

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    Gosh yes should've known better, thanks.

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    Registered User timkel's Avatar
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    Quote Originally Posted by wskteo View Post
    Thanks, I figured there were some here that knew. Also, why are there 2 holes in the front on the vintage upper, and the receiver pin hole seems larger, is it ok if I made my lower pin hole bigger?
    The smaller hole is for the barrel pin. There should be a roll pin pinning the barrel in place.
    The larger receiver pin hole should be the same size(0.25"). But with a recess on the end for using a "B" type take down pin.
    "B" type pin pic

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