Be Wary of used AR Upper Receivers

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  • calicojack

    American Sporting Rifle
    MDS Supporter
    May 29, 2018
    5,348
    Cuba on the Chesapeake
    I bought a used stripped Colt upper receiver back in March. It came from a private individual on Gun Broker. I got around to working on an upper for it when I noticed a hairline crack in it. Guess I could give it away as a paper weight, but that's about it. :mad54:
     

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    Sundazes

    My brain hurts
    MDS Supporter
    Nov 13, 2006
    21,287
    Arkham
    I doubt the seller will do anything, you have had close to 6 months. Still sucks though.
     

    ToolAA

    Ultimate Member
    MDS Supporter
    Jun 17, 2016
    10,499
    God's Country
    Jb weld and some extra duct tape. Not that it will actually help, but it’s sort of like wearing a mask at an outdoor soccer game. You want to appear responsible. And, It will still last longer than a 3D printed lower.
     

    Magnumite

    Ultimate Member
    Dec 17, 2007
    6,561
    Harford County, Maryland
    The stop drill is a good idea. I realize is a lot going on dynamically in that part of the rifle when firing. It seems the compression from the barrel nut and barrel extension should keep the crack area stable. Argument there is, if its stable why did it crack?

    Inclusions, etc aside, caused by the sharp corner made when the extension pin recess was cut into the receiver. I’ll take a tiny file and start putting a slight round on the corners of that cut in future receivers I build on. I’ll roll the file rather than use longitudinal cuts to eliminate stress risers…like I do in 1911’s where needed:

    From the op’s pic, it looks like metal was displaced outward by the extension pin. Maybe the cut was slightly narrow…causing a tension stress at the corners.
     

    Bountied

    Ultimate Member
    Apr 6, 2012
    6,884
    Pasadena
    The stop drill is a good idea. I realize is a lot going on dynamically in that part of the rifle when firing. It seems the compression from the barrel nut and barrel extension should keep the crack area stable. Argument there is, if its stable why did it crack? Inclusions, etc aside, caused by the sharp corner made when the extension pin recess was cut into the receiver. I’ll take a tiny file and start putting a slight round on the corners of that cut in future receivers I build on.

    Probably over torqued the barrel nut and the defect was exposed. Also could be from removing a flash hider and not using a lug style vise. Too much torque on the index pin going sideways could do it?
     

    calicojack

    American Sporting Rifle
    MDS Supporter
    May 29, 2018
    5,348
    Cuba on the Chesapeake
    Probably over torqued the barrel nut and the defect was exposed. Also could be from removing a flash hider and not using a lug style vise. Too much torque on the index pin going sideways could do it?

    I was wondering the same. The index notch has a couple indentations that may indicate the barrel was twisted with a fair amount of torque.
     

    smokey

    2A TEACHER
    Jan 31, 2008
    31,412
    Probably over torqued the barrel nut and the defect was exposed. Also could be from removing a flash hider and not using a lug style vise. Too much torque on the index pin going sideways could do it?

    Yup
     

    easy

    Member
    Nov 25, 2011
    81
    SEOK
    Correct me if I'm wrong here. I was under the impression that drilling of a stop hole on aluminum did little to nothing to stop the crack from growing. On steel, yes it works.
     

    teratos

    My hair is amazing
    MDS Supporter
    Patriot Picket
    Jan 22, 2009
    59,775
    Bel Air
    I did this to a BCM trying to true it. Got cool and the lapping tool caught. I didn’t sell it…I donated it to SOTAR.
     

    Magnumite

    Ultimate Member
    Dec 17, 2007
    6,561
    Harford County, Maryland
    Probably over torqued the barrel nut and the defect was exposed. Also could be from removing a flash hider and not using a lug style vise. Too much torque on the index pin going sideways could do it?

    Barrel nut has a minimum torque of 30 footpounds, typically ends up greater to align for the gas tube. The flash hider has an absolute maximum of 25 footpounds (which to many is too much, myself included) on a shorter moment arm. I highly doubt putting torque on the flash hider caused the crack.

    The cause of this was pre-barrel nut torquing. Maybe the recess for the extension alignment pin was shallow, that could put a stress riser in the receiver from torqing the barrel nut. Any torque cause would tend to pull the receiver area toward the nut and separate it.
     

    John from MD

    American Patriot
    MDS Supporter
    May 12, 2005
    22,728
    Socialist State of Maryland
    Correct me if I'm wrong here. I was under the impression that drilling of a stop hole on aluminum did little to nothing to stop the crack from growing. On steel, yes it works.

    Drilling a stop hole reduces the stress at the tip of the crack. It doesn't matter what the material is as long as it is not something soft and flexible like silicone.
     

    calicojack

    American Sporting Rifle
    MDS Supporter
    May 29, 2018
    5,348
    Cuba on the Chesapeake
    Maybe the recess for the extension alignment pin was shallow, that could put a stress riser in the receiver from torqing the barrel nut. Any torque cause would tend to pull the receiver area toward the nut and separate it.

    You can't see it in the photo, but there are indentations on either side of the index slot that support that theory.
     

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