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Thread: SIG 552 and 553 operating systems - a photo detail

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    SIG 552 and 553 operating systems - a photo detail

    With all the recent SIG 55x discussion, the topic of the 552 vs. 553 operating system was brought up. I figured I would put together a photo comparison detailing the two operating systems. Please let’s not turn this into a painful debate comparing the 55x series to the M4, the domestically produced 556, or other platforms. This is merely a photo overview of the mechanics of 552 and 553 operating systems. I am making a few assumptions throughout this comparison…namely that the reader is familiar with the 550/551/556 operating system. I will also refer to the 550/551 and 550/551/553, assuming that the reader understands that the 556 is based upon the 551 operating system.

    Before I start, please excuse some of the photos….I recently purchased a new camera and cannot for the life of me figure out the macro setting yet.

    First, a gratuitous photo of my 556-turned-552-turned 553. This started out life as an early Sigarms 556 which was sent to Frank Hatten at Antique and Modern Arms. He modified the receiver by opening up the trunion to 552-spec, then modified the barrel to 552-spec, and finally welded on a Swiss steel optics rail that had been removed from a demilled 552 parts kit. To turn this into a 552 clone, I used a brand new 552-2 parts kit purchased from Larry Gaglio when he owned Capital City Firearms. It also wears a Swiss Arms manufactured railed forearm that I was lucky enough to purchase from Larry Gaglio a few years ago. All 553 parts were purchased from Mike at Colorado Gun Sales.



    Next, a quick comparison of the gas piston heads and gas regulators of the 551, 552, and 553 to illustrate that the 552/553 operating systems use a piston that is smaller in diameter than that of the 550/551.

    The 553 gas piston head outer diameter is detailed in the first photo. The second photo details that of the 552 gas piston head. Notice they are identical. The third photo details the inner diameter of the 553 (and 552) gas regulator.





    The 551 gas piston head outer diameter and inner diameter of the 551 gas regulator are illustrated below.





    The 552 operating system more closely resembles the AK than its bigger brothers the 550 and 551 in that the gas piston is semi-rigidly fixed to the bolt carrier and the return spring and return spring guide rod are behind the bolt carrier. More on the semi-rigidly fixed to the bolt carrier later.

    The 553 operating system is essentially a scaled down version of the 550/551 operating system, with the gas piston being surrounded by the return spring. Also like the 550/551 the gas piston/return spring is linked to the bolt carrier by the charging handle. The 553 uses the same charging handle as the 550/551. Notice that the return spring is a flat coiled type, whereas the 550/551 uses a braided music wire type spring. The coiled spring seems to have been used in the 553 to provide maximal length of compressability when in full recoil/compression. As previously mentioned, the outer diameter of the 552 and 553 gas piston is smaller than its counterpart in the 550/551 series.




    Also notice that the charging handles are different (and not interchangeable) between the 552 and 550/551/553. The interface with the bolt carrier and the charging handle is unique on the 552 in that it structurally does nothing more than offer a means to charge the rifle. On the 550/551/553 system the charging handle actually mechanically links the piston/return spring to the bolt group. The 552 parts are on top, the 553 parts on bottom.




    Last edited by JoshNC; 03-27-11 at 19:47.

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    Next, the interface of the 552 gas piston and bolt carrier is detailed. The first photos are of a detail-stripped 552 bolt carrier. This is not something one would do for routine maintenance. Notice what I will call the locking flange protruding from the bottom/rear aspect of the stainless steel gas piston. Next, notice the recess cut out of the top/rear of the bolt carrier where this locking flange interfaces. There is a roll pin just below the recess which functions to keep the gas piston’s locking flange from rotating (and thus keeps it fixed in its position of interface with the bolt carrier). Note that the roll pin does NOT ever pass through the locking flange of the gas piston; it merely prevents the gas piston from rotating out of the recess in the carrier. There is a slight amount of fore/aft play even in a brand new, never fired 552 gas piston where it interfaces with the bolt carrier. This is ever so slight, as in a hair’s width. However, it is in this location that the 552 operating system has been known (in rare instances) to fail. With time, the locking flange of the gas piston is peened on its rearmost surface, where it impacts the front face of the recess cut out in the bolt carrier. This peening has been the case on all specimens of 552 carriers that have come through my hands (n=3…not scientific, but I am just reporting what I have personally seen). You can see this on the second to last photo detailing the interface between the piston and carrier. There have been rare reports of carriers fatiguing and experiencing catastrophic failures at the recess cut out of the bolt carrier. One individual with personal experience as it relates to these failures is a member of this site, so I will let him chime in should he choose. This seems to be with high round count fullauto rifles, as I have corresponded with a gentleman in Switzerland who has two 552s, both very high round count semiautomatic rifles (he has had to replace both barrels once) and he has not had any failure of his bolt carrier. Similarly a personal friend of mine is the LE/Mil distributor for SAN/Swiss Arms throughout the Caribbean and has sold many a 552 to drug interdiction teams in that part of the world. He reports their rifles to be used regularly (I have no figure on round count) but only in semiauto, and that they have not had any failures of the bolt carrier. Again, not my personal experience so take it for what it is worth.









    Next, is the difference between the 552 and 550/551 trunion. This is a critical small detail. The 553 exists in two mechanically slightly different formats. There are the 553s that have been made from existing 552s and de novo 553s (which use the same receiver as the 550/551). The 552-553 conversion is accomplished by means of a special 552-to-553 conversion kit, consisting of a slightly modified 553 gas piston/return spring and a slightly modified 552/553 gas tube. The de novo 553s use a standard 550/551 receiver, unmodified 552 gas tube, and standard 553 gas piston/return spring assembly.

    One need first to understand the differences between the 550/551 trunion and the 552 trunion. I apologize in advance, the 552 trunion photos were taken with my new camera thus the macro quality sucks.

    The 552 trunion is on the left, the 550/551 trunion on the right.





    The 552 trunion looking head on.





    The 550/551 trunion. This has been removed from a demilled receiver front end. The gold color is the furnace brazing that is done to join the trunion with the external sheet metal in addition to the spot welds on Swiss receivers. These are older photos taken with my previous camera that went TU when my wife dropped it into the sand at the beach…notice the better macro quality.






    Notice that the 552 trunion has been milled out so that the gas piston (which is fixed to the bolt carrier) can slide in from the rear through the recess opened in the trunion. This has obvious consequences in that there is now minimal residual steel in the trunion where the 552-553 gas piston/return spring assembly can interface to allow compression of the return spring.

    A comparison of the 552-553 and de novo 553 operating systems is detailed next. The gas piston/return spring assemblies can be seen below. The 552-553 conversion assembly is seen on top and the de novo 553 below. Notice that there is a steel flange at the rear of the assembly. This is what abuts/interfaces with the trunion to compress the return spring during operation.




    Again, the 552-553 conversion assembly is on top and the de novo 553 assembly on bottom. Notice that the 552-553 conversion assembly has a steel flange that is larger in width and diameter than that of the de novo 553 gas piston/return spring assembly.







    The gas tube of the 552-553 conversion kit differs slightly from the standard 552 gas tube. Of note, the de novo 553 uses a standard unmodified 552 gas tube. Both variants use the standard 552 gas regulator. Below the standard 552/de novo 553 gas tube is on top and the 552-553 conversion gas tube is on bottom.






    A caliper is used to better illustrate the difference. Note that the rear face of the gas tube has simply been milled down on the 552-553 conversion gas tube. This is to accommodate the flange on the gas piston/return spring assembly which is larger in width and diameter than that of the de novo 553. Notice that the larger flange will not fit inside the gas tube. This means that disassembly of the 552-553 conversion gas system requires one to completely remove the gas tube in order to remove the piston/return spring assembly.
    The standard unmodified 552/de novo 553 gas tube.





    The 552-553 conversion gas tube.




    That about does it. I hope this has been informative.
    Last edited by JoshNC; 04-21-12 at 08:54.

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    Outstanding.

    Thanks for taking the time to post this!
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    Wow, fantastic post (worthy of a sticky in my opinion)!

    Thanks for taking the time to put this together.

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    Question: which action is used in the USA 556 pistol and 556 SBR?
    Kein Mitleid Für Die Mehrheit
    What Happened to the American dream? It came true. You're looking at it.

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    Thank you.

    Very informative.
    It's hard to be a ACLU hating, philosophically Libertarian, socially liberal, fiscally conservative, scientifically grounded, agnostic, porn admiring gun owner who believes in self determination.

    Chuck, we miss ya man.

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    Quote Originally Posted by variablebinary View Post
    Question: which action is used in the USA 556 pistol and 556 SBR?
    It uses a gas piston with the same diameter as that of the 550/551, but it is about a half inch longer than the 552/553 gas system so as to allow for the larger diameter piston and the standard braided music wire return spring.

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    thanks for taking the time to post this.

    great info there.

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    Thanks Josh. Those are the best photos I've seen anywhere of the 552 and 553 upper internals.

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    Josh - Thanks for the pics. I tried to explain it with words, but pictures really are worth a 1000 of them!
    " In War, Strife, and Acts of Bravery, it's always the strong one's that get taken from the GENE POOL......and we wonder why society is getting weaker!!!" - by me

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    Hey thanks for the kind words guys. Glad it has been educational.

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    Nice writeup, looks like Kisara is back too.
    "Life is short, but the years are long." - Robert A. Heinlein

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    Excellent post!!!
    Last edited by Dirtyboy333; 05-04-11 at 01:57.

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    Holy extensive detail, Batman!

    Fantastically informative.

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    Thank you for your time and effort in compiling a very detailed explanation for all of us JoshNC. With the release of the 60 SIG 552's from Midwest Distributors, this thread is going to be used by alot of us.

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    Rico Ghinolfi, Swiss Arms gunsmith, explains the design features of the 55X :


    http://www.youtube.com/watch?v=Gy05V...layer_embedded

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    Quote Originally Posted by SkyPup View Post
    Rico Ghinolfi, Swiss Arms gunsmith, explains the design features of the 55X :


    http://www.youtube.com/watch?v=Gy05V...layer_embedded
    very nice......I asked this in the comments section on YT but i'll ask here also: On that new 553 CH at the end of the vid, is it using the same stock as a 556 or is it a much higher quality that looks similar? If its different, are they available (or going to be) in the US?

    Interesting that the Swiss/Germans are using the aluminum lower instead of the steel.

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    Quote Originally Posted by Dirtyboy333 View Post
    very nice......I asked this in the comments section on YT but i'll ask here also: On that new 553 CH at the end of the vid, is it using the same stock as a 556 or is it a much higher quality that looks similar? If its different, are they available (or going to be) in the US?

    Interesting that the Swiss/Germans are using the aluminum lower instead of the steel.
    That stock is made in Switzerland and is folding/collapsing. It was debuted around 2007/2008, I believe at the European IWA convention. The stock that comes on the 556 is a knock-off of the Swiss stock and made in who-knows-where....definitely not Switzerland.

    The first 556 classics came with Swiss made folding/collapsing stock. I snagged two of these back when the Classic was first released. I have not seen a Swiss model in some time. Like all things SwissArms, mike at CO Gun Sales should be able to get it for you. But it will cost you; I believe Swiss cost is close to 400 Swiss Franc. Do a Google search for Colorado Gun Sales and call to inquire.
    Last edited by JoshNC; 06-06-11 at 07:39.

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    Thanks Josh. I think I remember seeing stocks like that on COGS website but I just wanted to make sure their different then what I already have.

    I like the looks of the non collapsing Swiss a lil more but I'm not sure what the pull is like. What setting would the pull resemble on a 556 stock? I like the middle the best but the lOngest isn't bad either.

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