The SIG SG 551 is a shorter version of the SG 550 and differs in that it has no bipod, and it can't be used to fire rifle grenades nor can it mount a bayonet.There is several versions of the 551 in existence, the ordinary SG 551, the SG 551 SWAT*, the SG 551 LB** and the SG 551 that was made for the US market that came with a 16" barrel. In all other aspects, refer to the SG 550 page.

*The SG 551 SWAT differs from the ordinary SG 551 in that some parts are made from corrosion resistant steel and some parts have a corrosion resistant coatings to better whithstand corrosive environments.

**The SG 551 LB (Long Barrel) that has the same barrel length and profile as the SG 550 from the gas block and forward. This version can of course launch rifle grenades and mount bayonets.

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I recently got a SIG picatinny rail and an A.R.M.S # 35 QD mount to replace the SIG STANAG mount seen above. The picatinny rail/A.R.M.S. mount is a better solution IMO, because it allows for a wide variety of scopes to be used. The STANAG mount is best suited for STANAG interface scopes (duh!), and scopes with a set magnification due to eye relief issues. SIG also offer varios other mount interfaces, see the Swiss Arms website for more info