Polymer lower update

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  • Chazp89

    Active Member
    Sep 1, 2017
    283
    I’ve been running a couple polymer 80% ar lowers for a while now. After an inspection I found something interesting...

    There’s a polymer 80 (gen 2) that has been used to build a 300 blackout carbine that has a little over 100 rounds through it and developed a hairline crack at the top of the buffer tube threads right where the charging handle comes out of the upper. (Pictured with stock and buffer in place)

    The other is a genesis cnc (with the brass threaded inserts) polymer lower that has about 200 rounds through it in 5.56 with both 16” and 20” uppers and developed a crack in the same place. (Pictured stripped).
    734102b1680f442be8c60711c3b82622.jpg
    933baf4f425a6197fc6ceaa5d3485949.jpg



    Sent from my iPhone using Tapatalk
     

    jrumann59

    DILLIGAF
    MDS Supporter
    Feb 17, 2011
    14,024
    Second one looks like a seam. At first glance thermal expansion may be culprit
     

    tallen702

    Ultimate Member
    MDS Supporter
    Sep 3, 2012
    5,102
    In the boonies of MoCo
    Second one looks like a seam. At first glance thermal expansion may be culprit

    It actually looks like the seam is a bit to the right of the crack looking at it.

    OP, I do believe the Poly80 will do a warranty replacement on this as it's not user error. Not sure about the Genesis one though.
     

    GuitarmanNick

    Ultimate Member
    Jan 9, 2017
    2,221
    Laurel
    That area seems to be a weak spot on most polymer lowers when used with rifle caliber upper. I have a Polymer80 lower that is mated to a 9mm, 10.5" upper. My thinking is that these may hold up if not subjected to the heavier rifle caliber recoil. With only about 200 rounds down the pipe, this is yet to be determined to my satisfaction.
     

    jrumann59

    DILLIGAF
    MDS Supporter
    Feb 17, 2011
    14,024
    It actually looks like the seam is a bit to the right of the crack looking at it.

    OP, I do believe the Poly80 will do a warranty replacement on this as it's not user error. Not sure about the Genesis one though.

    Maybe on the top one but the crack in the genesis is too clean. Either way the OP should go back to the manufacturer.
     

    1time

    Ultimate Member
    Apr 26, 2009
    2,258
    Baltimore, Md
    That area seems to be a weak spot on most polymer lowers when used with rifle caliber upper. I have a Polymer80 lower that is mated to a 9mm, 10.5" upper. My thinking is that these may hold up if not subjected to the heavier rifle caliber recoil. With only about 200 rounds down the pipe, this is yet to be determined to my satisfaction.



    Unless you find a DI 9mm, it is not lighter recoil or less jarring to an upper than 5.56.
     

    dannyp

    Ultimate Member
    MDS Supporter
    Oct 30, 2018
    1,463
    not to derail the thread , other than weight . why would anyone buy a poly lower ? there are plenty of metal 80%'s out there . thanks , dan
     

    tallen702

    Ultimate Member
    MDS Supporter
    Sep 3, 2012
    5,102
    In the boonies of MoCo
    not to derail the thread , other than weight . why would anyone buy a poly lower ? there are plenty of metal 80%'s out there . thanks , dan

    Ease of machining. You can do a poly-80 with a dremmel and semi-okay tooling. Messing with aluminum is a bit harder if you're not running an actual mill.
     

    1841DNG

    Ultimate Member
    Apr 17, 2016
    1,143
    not to derail the thread , other than weight . why would anyone buy a poly lower ? there are plenty of metal 80%'s out there . thanks , dan

    Pretty much the plastic is a lot easier to deal with than metal if you don't have the proper tools.

    I hear that that area is the more likely failure point on those, but I have never made one so I am perfectly open to being corrected. Now if only they made a GWACS style 80%. That would be appealing to me.
     

    lazarus

    Ultimate Member
    Jun 23, 2015
    13,673
    Pretty much the plastic is a lot easier to deal with than metal if you don't have the proper tools.

    I hear that that area is the more likely failure point on those, but I have never made one so I am perfectly open to being corrected. Now if only they made a GWACS style 80%. That would be appealing to me.

    A GWACS 80 and I’d buy 10.

    I have a couple of their lowers and love them. Super strong, nice and light. Perfect LOP for me.
     

    GuitarmanNick

    Ultimate Member
    Jan 9, 2017
    2,221
    Laurel
    Unless you find a DI 9mm, it is not lighter recoil or less jarring to an upper than 5.56.

    I think you mean lower, in which case I must disagree.

    In my experience with two 10.5" uppers, one in 5.56 and one in 9mm, there is a noticeable difference in the recoil I feel which means there is certainly a difference in energy transferred from the uppers to the lowers.

    My 9mm build uses a heavier spring and buffer which reduced the travel of the bolt and therefore the energy transferred to the buffer threads on the lower. It is a matter of physics.

    Since most pistol caliber cartridges will require a shorter bolt stroke, a similar reduction in the energy transfer should be seen when the proper spring and buffer are in place.

    I would also avoid any or dropping any polymer AR. The material is fairly strong, but it is far easier to damage than aluminum.
     

    1time

    Ultimate Member
    Apr 26, 2009
    2,258
    Baltimore, Md
    I think you mean lower, in which case I must disagree.

    In my experience with two 10.5" uppers, one in 5.56 and one in 9mm, there is a noticeable difference in the recoil I feel which means there is certainly a difference in energy transferred from the uppers to the lowers.

    My 9mm build uses a heavier spring and buffer which reduced the travel of the bolt and therefore the energy transferred to the buffer threads on the lower. It is a matter of physics.

    Since most pistol caliber cartridges will require a shorter bolt stroke, a similar reduction in the energy transfer should be seen when the proper spring and buffer are in place.

    I would also avoid any or dropping any polymer AR. The material is fairly strong, but it is far easier to damage than aluminum.

    I did mean lower and I disagree with your disagreement. The buffer is heavier because the BCG is heavier. They both need to be heavier to deal with the blow back operation as opposed to the DI operation of the 5.56 where the bcg is lighter and can be extremely light and still work. Companies like lone wolf and sig both came out with short stroke piston set ups trying to keep their 9mm carbines flat shooting.

    Explanation from LWD:
    The AW-PCC is an AR-15 model platform which sports a rotating, locking bolt operating system, side charging, short stroke gas piston system instead of a straight blowback design. The short stroke gas piston system eases recoil with the reciprocating mass resulting in quicker follow up shots, compared to a heavier, rearward force, blowback system. The AW-PCC uses a standard AR/M4 fire control group for ease of training. The attached buttstock is adjustable with a mil-spec tube. These combined features produce an absolute reliable, cleaner, quieter, smoother and structurally sound carbine.
     

    Rambler

    Doing the best with the worst.
    Oct 22, 2011
    2,162
    It actually looks like the seam is a bit to the right of the crack looking at it.

    OP, I do believe the Poly80 will do a warranty replacement on this as it's not user error. Not sure about the Genesis one though.

    A "seam" would imply that 2 parts are joined at that interface. What you are seeing is a mold-line or flashing produced in an injection molded (or cast) part where the mold pieces contact each other. i.e. "parting lines"

    The above does not mean that a parting line cannot be a weak point. Injection molded parts can have problems due to uneven mold heating and part cooling which can cause stress concentrations at certain locations in the part including parting lines. This uneven heating/cooling issue is one of the reasons some polymer receivers were out of spec on the take-down pin holes and other features. The molds were correct but the temperatures were incorrect and the part distorted.
     

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