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Thread: AR9s and 147 Grain Ammo

  1. #1
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    AR9s and 147 Grain Ammo

    I have an FM Products FM9 that is all factory parts except the muzzle brake. Recently FM Came out with a troubleshooting/break in guide. In it, for defensive ammo, they recommend 124 grain HST. Of course, all of my defensive ammo is 147 grain.

    I have noticed my AR9 has a little more trouble feeding and cycling 147 grain ammo. Most of what I shoot is reloads and I typically go mid-range on the powder/power level. However, even when I go to full power the 147 grain loads don’t always lock the bolt back on the last round (new Magpul 12-round magazine) and have sometimes some Have trouble feeding.

    I noticed that the recommended round, HST 124 grain has a higher muzzle energy than the 147 grain HST; does that translate into more force to cycle the action on a blowback like this? If that’s the case, if I reduce the weight of the recoiling parts or reduce the spring force, would that help reliability?

    Thanks.

  2. #2
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    Since 147 gr is subsonic, trying a different recoil spring would help. I shoot only subsonic in my AR 9 since I shoot it suppressed.


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  3. #3
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    The 147 shape is a little more troublesome to feed and has lower muzzle energy.

    In a simple blow back, this directly translates to less energy to cycle the action.

    Reducing the bolt mass will speed up cycling.


    Quote Originally Posted by JB2000 View Post
    I have an FM Products FM9 that is all factory parts except the muzzle brake. Recently FM Came out with a troubleshooting/break in guide. In it, for defensive ammo, they recommend 124 grain HST. Of course, all of my defensive ammo is 147 grain.

    I have noticed my AR9 has a little more trouble feeding and cycling 147 grain ammo. Most of what I shoot is reloads and I typically go mid-range on the powder/power level. However, even when I go to full power the 147 grain loads don’t always lock the bolt back on the last round (new Magpul 12-round magazine) and have sometimes some Have trouble feeding.

    I noticed that the recommended round, HST 124 grain has a higher muzzle energy than the 147 grain HST; does that translate into more force to cycle the action on a blowback like this? If that’s the case, if I reduce the weight of the recoiling parts or reduce the spring force, would that help reliability?

    Thanks.
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  4. #4
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    Mine cycles noticeably more vigorously with lighter projectiles.
    RLTW
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  5. #5
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    I have a Colt style dedicated lower and use it with a 5” barrel with suppressor. It has a heavy 9mm buffer and one of the tubbs flat wire springs. Even with the original RRA 9mm buffer I started with I never had a problem with 147gr, both FMJ-FP and HST HP. In fact, I have only put a few 115 gr through it. That was just to see the difference in report between the standard and subsonic rounds.

  6. #6
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    Thanks for the replies everyone. There is a 2 Oz steel insert in the carrier. I’ll remove that and try it with my 147 grain loads. If it’s too “vigorous” I can drop it right back in.

  7. #7
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    I have seen the same issues with my 8.5" FM9 and a Spikes 9mm buffer with Ranger 147g.

    To fix feeding I polished the bottom of the chamber and ramp until shiny. After that I got a Failure to go into Battery with only 147 I believe because of less oomph to push the buffer/spring back and hence forward.

    To fix that I polished the leading edges of the AR type extractor to make it easier to clear the case rim upon feeding. Note the Colt 9mm extractor uses a much narrower and smoother leading edge bevel.

    147 is my duty load so I have a bunch but I was tempted to buy some SD 124+P just for my 9mm PCCs... Then I realized that's what I have 5.56 guns for!

    Dennis.
    Last edited by Dennis; 09-24-20 at 23:40.

  8. #8
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    Quote Originally Posted by JB2000 View Post
    Thanks for the replies everyone. There is a 2 Oz steel insert in the carrier. I’ll remove that and try it with my 147 grain loads. If it’s too “vigorous” I can drop it right back in.
    What sort of spring does the gun have? I have found that I can use a lot of mass with no issues, but increasing spring weight causes short cycling, even with pretty average cycling mass. My current setup has a tungsten weight in the bolt. Colt spring.

  9. #9
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    It has a .308 carbine spring. My other thought was to try a 5.56 carbine spring. Just not sure which change is a bigger change. I will probably take all of the components to the range and try them out with factory ammo.

  10. #10
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    Took my FM9 out over the weekend and shot some 147 gr reloads and 147 Winchester Ranger RA9T. It recoiled “briskly” with the 2oz weight removed from the carrier and I didn’t have any lock back issues.

    However, it wouldn’t feed the Rangers very well. Both the 147 gr Rangers and some 147 gr. XTPs I loaded caught on the “feed ramp.” Both of those are more cone-shaped bullets so when I can find 147 gr. bullets with a rounded profile I’ll try those.

    In the meantime, I’ll look for some 115/124 gr. hollow points to try. FM recommends standard pressure 124 gr. HSTs.

    By the way, why is there no ramp on a 9mm AR barrel? It seems like that would greatly improve reliability.

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