I'm still not sure if her 552 and 556 variants count as sisters.

ezekill said:
I'm still not sure if her 552 and 556 variants count as sisters.

Honestly, I don't think so. Sig herself said she doesn't have any variants. Also, in my own experience, the Sig SG 552 and 556 are practically the same. The majority of their parts, such as barrels, receivers, gas systems, and furniture, are interchangeable. The only major difference I can see is the lower receiver. The 550, 551, 552 and the like have similar lowers which take the Sig 5.56 magazine. The 556 is a STANAG magazine variant and is the only California legal version. Even so, with a 556 upper and lower, it is entirely possible to find parts for a 550 and 551 designed to fit the 556 in order to make it as close to the 551 as possible. Also, with this in mind, you can find a 551 lower that you could swap out with so your 556 will take the Sig 5.56 magazine with a few adapter parts. Since the upper receiver is legally considered the gun itself, as long as you have the money, you can buy any Sig SG 550 variant and change its parts to technically convert it into another variant. Full auto and 3 round burst capability is possible, but more difficult to find. Also, you'll have to register that gun as an assault weapon.