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9mm AR build question

Discussion in 'Rifle Country' started by unlimited4x4, Oct 10, 2016.

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  1. unlimited4x4

    unlimited4x4 Member

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    I'm confused on the buffer needed for the 9mm AR. Reading it appears the heavy buffer 5.3 oz is preferred, but then there 9mm specific buffers in the 7 to 9 oz range. I understand that 9mm AR's can be touchy and may need some tweaking with different buffer and spring combos, but I'd like to start with the combo that is most likely to work. So, do I start with a typical rifle heavy buffer and use a buffer spacer or go straight to the 9mm specific buffer?

    Also, the buffer spacers like spikes sells. Are these only used when using rifle length buffers and not the elongated buffers made specifically for the 9mm?

    Thanks
     
  2. GunnyUSMC

    GunnyUSMC Member

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  3. Arizona_Mike

    Arizona_Mike Member

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    See below. Double posted when I tried to edit.
     
    Last edited: Oct 10, 2016
  4. ssyoumans

    ssyoumans Member

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    I'm running a Spikes Tactical ST-9X 9mm stainless steel buffer I got off AimSurplus for $50.
    It runs flawless with all my rounds, from 90gr JHP to 165gr subsonics.

    I'd suggest picking up a dedicated 9mm buffer.
     
  5. Arizona_Mike

    Arizona_Mike Member

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    I've got a few posts on my build and issues including feed and disconnector problems. There is no 9mm mil spec and a lot of parts out there don't work well in the wrong combinations. Upper receiver/bolt to trigger tolerance and block/mag match are the issues I ran into.

    The long buffer is to keep the bolt from going back so far that it gets a running start against the LRHO. Some people have experienced LRHO catch breakage or even receiver damage with standard buffers. Personally I always run long buffers.

    Spring is not that important in a blowback. Weight is. You need to delay ejection as long as possible and only inertia can do that. Due to Hooke's Law unless it is a high spring pre-tension design (which the AR is not), springs exert very little force in their first few mm of travel. You want the heaviest bolt and buffer you can get.

    Here is a good article on blowback theory: http://www.orions-hammer.com/blowback/ Most successful blowback designs have a bolt/slide velocity of about 4m/s. Even with an M16 Bolt and tungsten long buffer, you are not going to get slower than 6m/s in an AR. The result is blowback ARs are hard on brass and shoot very dirty.

    If you don't want to spend the dough on a big tungsten 9mm buffer, go with the Spike's ST-9X stainless steel. It is almost 8 ounces. That plus a 12oz bolt will get you to about 20oz. I find that works reasonably well--I do get the occasional bulged case and I get a lot of fouling in the gun. I'm not a huge fan on the 9mm AR platform--I built one because I have a stamped SBR lower.

    Mike
     
    Last edited: Oct 11, 2016
  6. Walkalong

    Walkalong Moderator Staff Member

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    I went with a spacer I made on the lathe.

    [​IMG]
     
  7. Arizona_Mike

    Arizona_Mike Member

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    That works.

    On a related note, I recently found a shorter-than-standard buffer on line to run an AR15 length tube on an AR10. I suspect it fits 2 slide weights. It appears to have a longer full diameter section to add weight.

    Mike
     
  8. jmorris

    jmorris Member

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    I picked up a model1sales "kit" and had no problems.
     
  9. Dentite

    Dentite Member

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    Great info posted by Arizona Mike. I also use and have good results with the spikes ST-9x 9mm longer buffer. Good weight to resist rearward bout movement and no need for the spacer. My 9mm bolt (quarter circle 10) has a weight in it as well.
     
  10. maxxhavoc

    maxxhavoc Member

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  11. unlimited4x4

    unlimited4x4 Member

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    Thanks all for the replies. So, it sounds like running a spikes ST-9x buffer plus a heavy spring would be a good start?
     
  12. wally

    wally Member

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    Some use $1.50 worth of quarters.
    :)

    Edit:
    Sorry, maxxhavoc beat me to it.
     
  13. rjrivero

    rjrivero Member

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    Since the 9mm bolt replaces a bolt plus bolt carrier, I'm kind of lost about your comment regarding using an M16 bolt. Can you clarify that for me?

    Now that you can get Tungsten Buffer weights from Geissele, I think you may be able to get to a 4 m/s standard if you so choose. I'll have to give that a whirl.
     
    Last edited: Oct 11, 2016
  14. Arizona_Mike

    Arizona_Mike Member

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    Brain fart. I obviously miss-remembered things from a coupe years ago. Perhaps my 9mm bolt was 12oz. Need to check . . .

    W weights add ~0.8oz each. Can be found for half the price of Geissele.

    Mike
     
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