In the ocean of AR Platform Rifles the SIG M400 seems like an unfinished design that was ripped off the drawing board and thrown into production.
The M400 has a few innovative features and many tried and true ones, but when I start to get excited about it I am disappointed. What I'm talking about is SIG integrated an ambidextrous magazine release into the lower receiver, which looks great and seems functional, but did nothing else to make it a true ambidextrous gun.
When I saw the ambi-mag release I immediately searched for an ambi-bolt lock/release... but there was none. Ok, well at least they made the simplest ambi convers..... WTF! They didn't even put an ambi-safety selector on the gun! What appears to be built from the ground up as an ambi is not at all. Of course you could always add after-market parts to achieve this, but then why did SIG waste their time on that neat magazine release and not ambi-fy the rest of the controls? I don't know either.
The markings on the receiver are block letters which look great. SIG also added a spring and plunger system to add pressure on the rear 'pin lug' to achieve a tight lock-up between the lower and upper. The QD-Sockets integrated into the receiver is a nice touch too.