[brawler1588]

what is the difference between these rifles and are they more like ar's if so are they better than other brands colt bushmaster ect. and body have one or the other like it or don't I already have the 556 are these better or worse than the 556 any input would be greatly appreciated. Thanks guys

 

[GUNSICK1]

Check this out. Did not find much on the 516.

SIG Sauer M400 | Shooting Illustrated

 

[Cossey]

The M400 is a normal direct impingement AR made by Sig. Nothing special.
The 516 is a gas piston AR. Somewhat special I suppose.
The 556 is a gas piston rifle similar to AR, but not quite.