I posted this in ther MR223 - and then noticed most of it pertained here.
The Sig 551 - I think BigRed saw a broken charging handle once, I like them, but they are heavy... The unit that had them swapped their 551's for M4A1's, as the 551 was not compatible for the Sound Suppressors and Night Fighting gear that are force multipliers in the modern age.
I think their are plenty of systems out there that have the potential to better than the M16FOW, however price tag of the coating, and materials used to design them are prohibitive in a general issue weapon.
The G36 system was a low cost gun forced on the German Army due to the costs with reunification. Its done abysimally in any external military testing.
Heck for a while you could see KSK running around Afghan with Diemaco C8SFW's... Until Tuetonic National Pride made the Hk416 a necessity.
Its not just how much more expensive than the M16FOW, its the entire lifecycle cost that just skyrockets with a new system.
The Colt M4 MWS as it comes from Colt with our M4RAS and the Matech BIS is around $1200 The entire infrastructure is set up around it - for 50 years.
A new weapon means new spare parts, new manuals, new training, new rifle racks, new parachute instructions.
I've seen numbers that support a 4-5x increase in costing over the actual system cost for system transition.