MAC 10 works in semi but not full auto.

Discussion in 'NFA Firearms and Accessories' started by Dan Forrester, Oct 2, 2017.

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  1. Dan Forrester

    Dan Forrester Member

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    Got a full size MAC 10 in 9mm with a UZI mag well welded on by "Practical Solutions" about 10 years ago. The gun runs great in semi auto. I can go through a magazine with no issues at all. Once I switch to full auto it maybe goes through 3 or 4 rounds and then jams up.

    At first I though it might be the magazine not feeding the rounds up fast enough. I have tried multiple magazines which I have taken apart and cleaned and re-assembled but no luck.

    I have two bolts both of which have been milled for UZI mags but same problem with both.

    Any ideas on what the issue might be?

    Thanks, Dan
     
  2. MachIVshooter

    MachIVshooter Member

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    Probably bolt too light and/or recoil spring too strong, cycling faster than the mag can feed. MACs have a ROF that is pretty borderline for conventional mags anyway, and if it's increased at all, I'd expect issues. If it feeds fine in semi, you know it's not a feed ramp issue, and as an open bolt gun, excessive bolt speed is really the only thing that makes sense.

    Can you weigh your bolt? If it's under 16 oz, definitely too light, and you'll have to add tungsten by milling or drilling due to the Uzi mag cuts. Should be around 19-20 oz, IIRC.

    This was an issue I fought a bit with my pack rifle build, a closed bolt semi with a 12.2 oz. bolt (including striker & charging handle). My bolt speed is around 40 FPS (calculated), and getting the thing to feed took a lot of careful hand profiling and polishing to make it feed on it's own. I actually had to make the bottom of the bolt kind of scoop shaped:

    IMG_20161028_131013093.jpg
     
    Last edited: Oct 3, 2017
  3. Dan Forrester

    Dan Forrester Member

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    I just weighed the bolt and it came out to 27 ounces. It is an older full size mac originally designed around the .45 acp but in 9mm. It is not the M11/9 which had the super fast cyclic rate. I specifically purchased the larger MAC in 9mm since it had a nice slow cyclic rate. It is easy to get off single shots with it in full auto.

    I appreciate the help!

    Dan
     
  4. jmorris

    jmorris Member

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    The Powder Springs MAC-10 9's came with milled, to reduce the weight, bolts (right/top) others were closer to the bar stock they originated from.

    image.jpeg
    In any case I would suspect the magazine(s) you are using or the load. Have you made any "improvements" like changing springs or buffer?

    FWIW the M-10 9 was rated at 1250 rpm the M-11 1200 rpm and the Uzi 600 rpm.
     
  5. Dan Forrester

    Dan Forrester Member

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    No changes to the spring or buffer. The bolt does not have any lightening cuts in it.

    Heres a couple pictures of the jams. This is with Tula steel case ammo and no suppressor. I believe it is 115 grain.
     

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  6. Dan Forrester

    Dan Forrester Member

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    These jams are with 158 grain Fiocchi and a suppressor attached. The jams look completely different. Like it is diving into the feed ramp.

    Thanks for the help!

    Dan
     

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  7. entropy

    entropy Member

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    Can't help but thinking there is a specific load you should be using wit the UZI mags; Seems like the Tula were coming too fast, and the Fiocchi too slow. The timing is off somehow, At lest the operator head space is OK. ;)
     
  8. jmorris

    jmorris Member

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    Interesting that it feeds, fires both fine in semi. In full does it make a difference if you have a full mag vs one with say, just 5 rounds?

    How many mags do you have for it and are any "better" than others?

    Have you tried the 115's with the can on?
     
  9. Dan Forrester

    Dan Forrester Member

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    If the Tula are too fast and the Fiocchi is too slow maybey I need to try something in the middle. I think I'll try some 124 grain 9mm NATO next time I'm out shooting.

    I have probably 20 or so mags. I believe they are German mags. I have tried multiple different mags. They all appear to be in good condition and the ones I'm using I have taken apart and cleaned.

    The mags hold 32 rounds and I rarely put more than 20 or so in them.

    Thanks for the help everyone!

    Dan
     
  10. Ranger Roberts

    Ranger Roberts Become a THR contributing member!

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    I know this sounds silly but on my brothers M11 you have to mash the trigger or it will do exactly what you described. I don't mean pull the trigger hard, I mean treat the dang thing like it owes you money. I know that it probably isn't a solution but I figured I would share my experience. My M11 on the other hand has no such problem.
     
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