552 info | SIG Sauer 556 Forum

552 info

Discussion in '556 Discussions' started by GiterDunn, Jun 18, 2020.

  1. GiterDunn

    GiterDunn New Member

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    Can anybody break down all the different variants of 552's for me, I've seen so many its confusing.
    I'm building a 552 look-a-like with 556 parts on a virgin upper, got my welds cleaned up so now its time to etch some rollmarks but I can't decide what they should be. For now this gun will run a 556 lower with some kind of pistol brace, but it will eventually be SBR'd with a swiss lower so I'd like the markings to be correct. Also this will be a sig grey painted gun so I want the rollmarks to be correct for that. So far all I've been able to figure out is that SP means semi-auto.

    I can only find one pic of a 552-1, it has a grey receiver and it is an SP gun. It's rollmark reads "SG 552-1 SP Commando Serial#" with no factory name or country markings on the line above. Most of the 552-2's I've seen are similar but have
    "SIGARMS Inc. Exeter-NH SAN Made in Switzerland Cal. .223 Rem." on the upper line, but I have seen one with no markings on the upper line.
    Another thing I noticed is that all of the 552-2's I've seen are grey receivers while all of the 552-2 SP's are black. The previously mentioned 552-1 SP is the only grey receiver SP I've seen.
    So I guess my question is what is the difference between the 552-1 and the 552-2?
    Also, were they all actually .223 or just the imported guns? I assume the swiss military 552's would be 5.56NATO.
     
  2. CGRBB

    CGRBB Active Member

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    -1 and -2 indicates the barrel twist rate. Forget which number is which twist though. Just because a gun is marked .223 doesn’t mean it is actually chambered in .223. There was a period when that was kind of a lost-in-translation thing, when it came to guns destined for certain markets. They may have been marked .223, but were 5.56 chamberings.
     
  3. CGRBB

    CGRBB Active Member

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    What do you mean by “virgin” upper?
     
  4. GiterDunn

    GiterDunn New Member

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    Oh, thanks for clearing that up. What were the 2 different twist rates?
    I'm making the upper almost from scratch. I bought a 552 trunion/barrel group a while back and have spent years trying to figure out what to do with it. I bought an airsoft gun to steal the classic stock and noticed all the metal on it is steel, so I gutted it for the receiver shell. Milled some bolt rails and bought a rear section off another forum and welded it all together. I've got it up and running now just needs cerakote and markings.
     
  5. CGRBB

    CGRBB Active Member

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    1 is 1/10. 1 is 1/7.

    Wow. That is an ambitious and unique project. Well done indeed.
     
  6. fachries

    fachries Member

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    +1
    [​IMG]
     
  7. GiterDunn

    GiterDunn New Member

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    Thanks, it was a lot harder than an AK build that's for sure lol. How can I tell what twist rate I have? I can't find any markings on the barrel.

    *nevermind* Just checked it by putting mark on a cleaning rod and watching how many times is rotated. 1.25 rotations in a 9" barrel makes it a 1:7 twist. Guess I'll mark it 552-2 then. Thanks for the help guys, I'll post some pics when I get it done.
     
    Last edited: Jun 19, 2020
  8. GiterDunn

    GiterDunn New Member

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    Another question, has anybody seen a 552-A1? I've read that the Swiss military were upgrading their 552's to the 553 recoil system and marking them as 552 A1. That's closer to what my build is, a 552 kit filled with 556 parts.
     
  9. CGRBB

    CGRBB Active Member

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    That is largely decided by what recoil system you are using. Are you using a 552 system (recoil spring and guide rod behind the carrier), or a 553 system (recoil spring wrapped around the piston, forward of the carrier)?
     
  10. CGRBB

    CGRBB Active Member

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    If your gun is using the 553 system, there’s really no sense in marking it “552” at all, as it isn’t one now, and wasn’t one previously.
     
  11. GiterDunn

    GiterDunn New Member

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    Well it was previously because that's what the kit was, and it is built to look like a classic 552 (no rails, just diopter sight), but the internals are just like a 553 only using 556 parts. I'm going to mark it as some sort of 552 because that's what it will look like and that's my dream gun, but now I'm considering marking it as a 552A1 because technically that's what it is, a 552 upgraded to 553 spec. I just wish I could find a pic of an A1 so I can get it right. The donor airsoft gun (ICS) had mostly correct markings except for the serial number, it's a shame they were destroyed when I plug welded the rails in. [​IMG]
    *edit* which proof mark is more correct, the BP (backwards B), or the NB inside a circle with crosshairs?
     
  12. CGRBB

    CGRBB Active Member

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    I believe the NB with crosshairs is the newer marking. If that’s the case, the BP would be more correct for an updated 552. I have never seen an image of a gun marked “552-A1”, but it certainly seems possible. However, I don’t know why an agency would bother remarking the guns, if they performed the update themselves. I guess SIG may have, if they were updated at the factory.
     
  13. CGRBB

    CGRBB Active Member

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    There are tons of diopter 553s, by the way. The irons-only configuration wasn’t exclusive to 552s.
     
  14. LKY13

    LKY13 Member

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    That airsoft receiver was not engineered to handle the stresses of a firearm.
    Similar folded steel receivers are fabricated with 4130 ordnance-- then heat treated.
    Lower grade steels can be utilized (see early communist block AKs) but at the very least the ejector location must be heat treated.
    I assume you welded-in the missing ejector? Hopefully you used a higher grade steel, and then heat treated this area?
    Did you have to compensate for any dimensions / tolerances that were "off" in the receiver when fabricating your receiver rails?
     
  15. GiterDunn

    GiterDunn New Member

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    Yeah my rails and ejector are milled from solid 4140, spot heat treated ejector like an AK. Heat treating the entire shell is unnecessary, they only do that for AK receivers because the fire control group pins will rotate and wear out their holes if they don't, not a problem for a Sig upper. IDK what kind of steel the airsoft shell is but it welded fine and didn't warp. It's just a hair thinner than my 556 receiver but thicker than AK receivers. Being taller and narrower than an AK receiver and having more complex bends makes it stronger even if it was as thin as the AK. I figure it being thicker and having full length rails to reinforce it it should be plenty strong, plus 5.56 is less recoil than 7.62x39 so it doesn't get loaded as hard.
    The inside dimensions of the airsoft receiver were perfect, length was good too. It being slightly thinner I guess the outside dimension is off but not enough to notice, it lines up with the tower of the lower just fine.
    I actually tried a molded carbon fiber receiver before this that extended past the trunnion to be a monolithic handguard. It was stiff as hell but I had trouble keeping it attached to the trunnion. The rails were bolted on like a screw build AK but I didn't want to drill and tap a 552 trunnion.
     
  16. GiterDunn

    GiterDunn New Member

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    [​IMG]
    Before welding the rails. It's all done now, ground flush and block sanded that whole side. That's why I was trying to figure out what rollmarks to but back on there. Turns out the airsoft marks were spot on except for the serial number being too long.
    [​IMG]
    Shot of the rails, ejector wasn't in yet here. The ground down screws at the back are to hold the sight on, it was aluminum so I couldn't weld it. This whole project is sort of a trial run to see if the airsoft parts are viable, if they are long term I plan on eventually refurbishing this gun and adding more Swiss parts, like a steel sight and rail. The first iteration of this gun isn't going to be perfect, there are some modifications left over from the carbon fiber upper project like holes in the rails and a lightened trimmed down gas block to clear the monolithic upper. So my 552 length handguards are going to need a 3d printed spacer for now. Something else I'm thinking about doing is buying an airsoft railed handguard to see how good it fits, I can't justify $500 for B&T. I've had very good results with airsoft stocks, I run them on all my Sigs. I reinforce them by splitting apart and epoxying everywhere they mate, then clamp them together very tightly and fill with expanding foam after the epoxy cures. Then trim down the mold lines and paint.
     
  17. CGRBB

    CGRBB Active Member

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    This is very interesting. I think it’s very cool that you’re doing this. I am a bit more of an uppity purist, so I’d never attempt this kind of project myself, but I damn sure respect you tinkering inclination.
     
  18. LKY13

    LKY13 Member

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    Yeah, the welds on the front trunnion look really rough-- but lots of respect for attempting all of this work yourself.
    And, for having the creativity / ingenuity to use an airsoft receiver as your base. Did you water jet cut the ejector?
    How did you ensure that the front trunnion and barrel were perfectly aligned with the receiver before welding? Did you fabricate your own jig?
     
  19. GiterDunn

    GiterDunn New Member

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    I hope one day I can afford to be a purist lol. I know about the welds it bothers me too, I gotta get a TIG. I just keep welding up and grinding down until there are no more porous spots, they don't look that bad now. Ejector was milled from flat stock, I got it roughly to shape and welded it in then put the whole receiver in the mill and trimmed it until the bolt cleared. My mill is a 1944 Van Norman with backlash and no DRO so it's all trial and error. Alignment jig was just a piece of flat bar milled to just larger than the bolt lugs and slide between the rails and into the trunnion, plus a lot of clamps.
    When I tried the carbon fiber receiver I 3d printed a soluble plastic mandrel that fit the rails and inside the trunnion tightly and matched the inside dimension of a Sig receiver. Degreased and surface activated everything (rails, mandrel, trunnion), filled any gaps with wax and vacuum bagged the carbon fiber directly onto it. After it cured drop it a bucket of water and the mandrel melts, perfectly smooth inside finish. I was relying on just epoxy bonding for the trunnion but it wasn't good enough. Took a hundred rounds but it eventually broke free. Rails stayed put because they were screwed also. I thought about screwing the trunnion but it would eventually egg out the screw holes like a screw build AK, but much faster cause it's metal on composite wear.
    Composite receivers will be a thing one day but it's gotta be a whole new trunnion designed for it, I'm working on a bolt action like that now.
     

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