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.