FNG needs advice
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  1. #1

    Default FNG needs advice

    I'm new to this forum and cartridge but have experience with the standard ARs. I recently built an uppper with a 20" barrel and cant get it to grab the second round. If I only put one in the mag it catches the bolt so I dont think its short stroking. It will manually empty the mag so I dont think its the mag. Using 95 gr factory loads, adjustable block and Anderson BCG. Any advice would be appreciated. Thanks

  2. #2
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    it really sounds under gassed to me. open up the adj block some more and test it
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  3. #3
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    Default FNG needs advice

    Its under gassed/short stroking.

    Place 1 round in the mag, charge and fire the gun. I'll bet the carrier is locking open on the face of the carrier and not the bolt.

    Lubricate your bolt body, cam pin and carrier rails with a good lubricant.

    1. Make sure your gas system is clear and aligned, including the gas block port with the port in the barrel as well as the gas tube port is aligned, free/clear with the gas block port. You can remove the set screw and use a light to verify.

    2. Try some Hornady 110-120 Ammo and see if it will function. The lighter Bullets and faster powder may be your culprit in allowing the 20" barrel to function. It's possible you may have to drill the barrel gas port to the next size up if it will not cycle the Ammo your currently using.
    Last edited by semi_cop; 03-12-2017 at 03:34 PM.

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  5. #4
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    I have had two 20" ARPs and the both started out short stroking. I changed from a full auto BCD to a semi auto and they ran fine. After about 100 rounds, switched back to the full auto BCD again and they are running good.
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  6. #5

    Default Update

    Opened the block all the way and it still wouldnt cycle, they were 90 grain ae varmint rounds. Got some 110 grain american gunners and it cycles like a champ. I'm hoping when I get enough brass to handload it doesn't have any issues or I might have to drill the gas port out a little, unless you guys think that is a bad idea. ARP 20" triad, the hole is a little smaller than my gas tube.

  7. #6
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    Heavier SPC II loads, or use of a different powder (once you handload) may negate needing to opening up the gas port. Also, as stated, after it's fully broken in it may behave better. My 18" ARP short stroked 85gr Barnes RRLP from SSA when new, but ate everything else. I never went back to see if it would eat them now (I should test next range date).

  8. #7
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    If you want it to cycle with a variety of loads, whether hand loads or commercial, from 90 -? you would probably be better off opening the port a bit once you exhaust all the suggestions given to you, including break in time. 20" barrels can be finicky if even one small issue with gas flow is present and sometimes drilling will compensate for something you simply can't find. If it indeed is locking the bolt back on the bolt catch when you fire one of those 90's one at a time you don't have a function problem though. Understand I said locked back on the bolt catch. In that case you no doubt have a mag problem with it feeding the next round with the 90's if it locks back single fired. I had a mag once where the neck of the case on light and shorter rounds would hang on a sharp edge of the mag as it left. Filing that sharp edge solved the problem. Sometimes something as simple as a defective mag spring will be the culprit. As said before, if it locks back on the bolt catch on single fire with the mag in place you are not short stroking.

  9. #8
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    I have one psa 25 round (d&h) that does this on a full mag. I can load 24 in it and it's great. Tried it on 3 different uppers, does it on all of them when loaded with 25.

  10. #9
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    I'd try tweaking the mag feed lips and spring before jumping in and drilling the gas port just to make sure its not a mag issue since you did say it's locking back on an MT mag

    try this thread for more helpful ideas http://68forums.com/forums/showthrea...al-information

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