[Corleone]

SIG 550 - Built-in bipods in the handguard. 534mm inner barrel, and quite a long weapon. Very nice for a 400-450 FPS semi-sniper.

SIG 551 - Shorter version of the 550. Although I haven't really checked too closely, I think all airsoft versions are replicas of the SG551-SB (Short Barrel) also known as the SWAT-version of the 551 in the realsteel community. It has a 363mm inner barrel and a handguard similar in design to the 550, just shorter.

SIG 552 - Short, sexy and mostly for CQB as it can get less precise over long distances. This is of course depending on what equip you get for improving accuracy (recommending Guarder clear hop-up, H-nub and a precision barrel). It has a 247mm inner barrel and a slighy different handguard design, mostly due to it's short length. In my opinion the best looking of them all! The holes in the handguard really makes it stand out.

The 552 can only take mini-size 8.4V conventional batteries, or a mini-type lipo, if you make some minor modifications to the gun. A least the TM 550, and ICS 551 use stick type batteries, maybe also the TM 551.

From what I've read, both the 550 and the 551 from TM has burst, but this info is only semi-confirmed. The 552 most certainly has not burst, but rather the more common full auto.

Jing gong make some good and cheap clones of the TM 550 and 552. For the most the JG has better internals than the TM, better meaning that the gears are stronger, reinforced gearbox etc.