You were good right up until this part. Pray tell, what is the torque spec for an A2 flash hider?
There isn't one. Muzzle devices are "timed" using a crush washer or shims between the device and the barrel shoulder.
EDIT, to the OP: Couple more things:
1), Do not attach your carry handle the way Sig did for this photo. The front of the carry handle should be flush with the front of the upper receiver. They have it one notch further back than it should be:
http://www.policemag.com/_Images/new...ws-SIGM400.jpg
Looking at the picture, it appears to be a standard AR15 DI variant. That being the case, as Iraqgunz has said, any 12/x28 threaded muzzle device should work.
A 3/4" box wrench should be used on the flash hider. Slide a patch or old t-shirt between the wrench and the muzzle device to prevent marking the finish if you wish.
The upper receiver or barrel should be properly secured in a vise while removing or installing a muzzle device.