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Form 1 Sig MPX

Discussion in 'NFA Firearms and Accessories' started by Madcap_Magician, Feb 6, 2019.

  1. Madcap_Magician

    Madcap_Magician Member

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    I have never done NFA guns before, and I'd really like to do an easy Sig MPX SBR off of a Form 1.

    Any owners want to chime in on their stock preferences, and any thoughts on the 4.5" barrel vs. tracking down an 8" gun?
     
  2. Ryanxia

    Ryanxia Member

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    As far as barrel length goes, I think ballistically the 4.5" will offer plenty for what you're looking for but check out Iraqveteran8888's video on 9mm lethality over distances. He does 100yds, 200yds & 440yds.


    Hope to see a picture when you have it built!
     
  3. Madcap_Magician

    Madcap_Magician Member

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    I'm actually thinking about 4.5" vs. 8" mostly in terms of gas pressure at muzzle, sight radius, rail space, and safety. A K-configuration gun would make it pretty easy to get your hand in front of the muzzle. Earlier I had been thinking about rail space, and whether a 4.5" rail would let me put a Magpul AFG on it, but if I Form 1 it, I suppose I can use an actual vertical foregrip.
     
  4. Dan Forrester

    Dan Forrester Member

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    I went with the collapsing stock but purchased the longer old style telescoping rods from Title II Arms. Verry happy with them and they only have two positions which is nice.

    http://www.titleiiarms.com/SIG_Telescoping_RODS.html

    Also have the original 8” barrel. I really don’t like the metric 13.5x1 threads and will probably get the barrel 3 lugged.

    I purchased a 13.5x1 metric thread adaptor for my liberty Mystic but it’s really awkward since the muzzle threads and suppressor adaptor threads are now in opposite directions. In other words every time you go to tighten down the suppressor you loosen the mount from the suppressor.

    For best results you probably need to either get it re-threaded to 1/2x28 or 3 lug the thing.

    Dan
     
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  5. MachIVshooter

    MachIVshooter Member

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    I would say which is preferable depends on whether you intend to suppress or not.

    No can, the 8" barrel will offer a little better velocity and a little lower noise at your ear.

    With can, the shorter barrel is going to keep velocities lower for better suppression and reduce your rifle's OAL.

    Though I have no personal experience, the gas operation of the MPX should make it one of the quietest, if not THE quietest, 9mm hosts.
     
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  6. Madcap_Magician

    Madcap_Magician Member

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    Thanks for the recommendation! I was thinking the 8" barrel would be my choice, and if/when I get to suppress, I'll go to a 4.5" barrel and get an aftermarket one threaded to 1/2x28 or with a three-lug mount.

    I'd like the option to suppress, but with the cost of a suppressor, cost of the tax, and Form 4 transfer times (since I don't have the ability to manufacture my own), I'd be looking at another $1,000+ dollars, so I am thinking I would prefer the 8", and if/when I get to suppress, I will buy a 4.5" barrel and retain the 8" handguard. If I went the other way round, I'd have the short barrel to maintain shorter OAL with suppressor attached, but I'd still need to buy a longer handguard. With an 8" barrel, I feel like I'd have more shootability for however long it is until I get around to getting a suppressor, probably years. Buying a new barrel is more expensive than buying a new handguard, but I am leaning toward the immediate shootability of the 8".

    Only downside... they're getting real hard to find since they got discontinued a year ago.
     
  7. MachIVshooter

    MachIVshooter Member

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    Do you have a small drill press? Because that's all you need to get going with a basic F1 can using solvent trap parts. And if you decide you aren't happy with the performance, you can have one of us SOTs professionally recore it for you. Another government shutdown notwithstanding, F1 waits are 2-3 weeks, and cost can really be as low or as high as you want it. There's no getting around the tax stamp, but the actual suppressor doesn't have to (and really shouldn't) be anywhere near $800, not even with a recore (unless you spend a bundle on all ti parts & cones that get replaced).
     
  8. Madcap_Magician

    Madcap_Magician Member

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    I don't even have a Dremel tool. :(

    Thanks for the tip on Form 1 times, that's one of the most attractive parts of an MPX SBR build. Easy peasy manufacture.
     
  9. MachIVshooter

    MachIVshooter Member

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  10. vtsteve

    vtsteve Member

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    I SBR’d mine with the factory collapsible stock and 8 inch barrel. On its own it’s a neat little package.

    It’s super quiet running subs, but it’s a bit cumbersome with a full size pistol can. I think a 4.5” barrel with the 8 inch hand guard would handle much nicer.

    These guys https://inleadwetrust.net/ sell some 3 lug barrels ready to drop in in various lengths. Factory barrels were impossible to find last I checked.
     
  11. Madcap_Magician

    Madcap_Magician Member

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    Thanks for the link! I am going to go with an 8" and buy an aftermarket short barrel if I get to suppressor buying.
     
  12. waterhouse

    waterhouse Member

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    753AF49A-8A3B-44D8-8933-81365DF1CADA.jpeg I have an 8”. I’ll probably get a shorter barrel at some point. It is our most fired non-rimfire gun. The kids love it.
     
  13. Madcap_Magician

    Madcap_Magician Member

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    What's the cheek and and shootability on the collapsible stock? I am going back and forth on what stock to go with.
     
  14. waterhouse

    waterhouse Member

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    Well, it feels like you are putting your cheek on a metal rod, but it is very shootable. It is a heavy 9mm gas gun, so there is virtually zero recoil. And to clarify a bit, the stock sort of touches my face but I wouldn’t say I really get a “cheek weld,” so much as bring the optic up to my eye line and shoot, and it just happens that the stock touches my face when I do that.
     
  15. MCMXI
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    MCMXI Contributing Member

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    I have the factory MPX SBR with the 8" barrel and didn't care for the twin rod collapsible stock so bought the folding, collapsible stock which I much prefer. The original stock would rip whiskers out of my face. I run a dedicated AAC Ti-RANT 9 on the MPX and with the 147s it's really quiet. I also swapped the trigger for a Timney and it's way better. The MPX is one of my favorite firearms to shoot.
     
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  16. Madcap_Magician

    Madcap_Magician Member

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    I bought mine and shot it a few weeks ago. It's a real pleasure to shoot. The only thing I can compare it to is a Ruger Mark pistol or 10/22. No recoil, relatively inexpensive shooting, loads of fun.

    I am leaning toward the side-folding minimalist stock at this point. I have the Sig PCB brace on there now, and it shoots well, so the minimalist stock is basically that. The Lage side-folder stocks look really good, though.
     
  17. MCMXI
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    MCMXI Contributing Member

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    Here's my MPX ... without a magazine inserted. :p

    sig_mpx.jpg
     
  18. Hoser

    Hoser Moderator

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    I also have a factory SBR with the 8 inch barrel.

    Very happy with the OEM stock. While the AR stock adaptors work well, they look terrible.
     
  19. zeke

    zeke Member

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    SBR'd a MPX by putting 4.5 in barrel, and a stock like MCMXI's. This stock can be lowered slightly to allow lower set open sights, and when folded it is a very compact package. If not planning on suppressed would keep the 8 in barrel, unless shortness is a priority.
     
  20. Madcap_Magician

    Madcap_Magician Member

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    I don't super love the folding/collapsible factory stock, I'm leaning toward either a Lage side-folding stock or the factory Minimalist Plus side-folding stock. Although the factory collapsible is tempting from a cool factor standpoint.
     
  21. MCMXI
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    MCMXI Contributing Member

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    @Madcap_Magician, if you want the factory collapsible stock send me a PM and we can talk. I have no use for it even though it is cool for sure.
     
  22. Madcap_Magician

    Madcap_Magician Member

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    I'll keep it in mind, but I don't want to have a stock in my possession until my stamp is approved, so hopefully in a couple weeks. :D
     
  23. Madcap_Magician

    Madcap_Magician Member

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    Got my approval back today! Filed 3/12, fingerprint cards I sent arrived at ATF on 3/22. Three weeks on the dot from submission date.
     
  24. MCMXI
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    MCMXI Contributing Member

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    Damn! That's fast!! Hope they get the Form 4 eForms up and running soon. I just sent the Form 4 paperwork (trust) in for a friend's factory DD SBR, and his suppressor shows up tomorrow so another Form 4 will be going out this week. He's probably looking at 8 months.
     
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  25. greyling22

    greyling22 Member

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    my cousin has a SBR mpx with a 4.5" barrel and a can. It is GASSY. we shot it next to my cmmg guard upper I built, and the guard was more pleasant. The 4.5" barrel was also not accurate for us at 50yds. It was missing the 8" gong about half the time, while we were pinging away with the 16" scorpion carbine and the 8" guard. The scorpion was the least gassy, but had the most recoil. The mpx and the guard were a tossup as far as recoil went.

    There was a thread over on ar15.com that referenced the owner of bowers suppressors talking about the 5" vs 8" guard, and he said the 8" suppressed better. But I was unable to find the scource of that comment, or the context that went with it.

    All 3 guns were better than a blowback ar9 in terms of recoil, gas, and suppression.
     
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