I'm a 516 fan, and really enjoy mine but you can't go wrong with either. Vwguy1994 & Johnnyloco pretty well covered it. Additionally, the 516 has a good (although heavy) quadrail, so one less upgrade to buy over the M400 and many other AR style guns. I like the clean operation of the piston system, but the 400 has the advantage of standardized DI gas system parts, so you trade some interchangeability for the cleaner, proprietary piston system. For optics, I currently run a Vortex Viper 4-16x44 on mine but would go with Johnnyloco's suggestions for your desired usage. Vortex is also coming out with the Razor AMG UH-1 holographic sight, that sounds real promising. Good luck and let us know what you decide upon.
Both are good ARs. As with others here, my choice was 516. I've also owned Colt's 6920 DI gun, but currently have only SIG & PWS' piston ARs here.
For me, an important difference is the gas control options provided by pistons rifles. Running a suppressed SBR indoors can be an uncomfortable experience - if you can't turndown the gas!!
Watering eyes and choking breaths aren't uncommon with typical suppressed DI rifles...
My suggestion with your rifle's optics is based on your home defense usage. I mean, optics for range fun guns can be almost anything you'd like to play with, and at very affordable cost.
However, IMO a weapon that may be used in a lifesaving situation has gotta be totally battle-proven. Those are EOTech, Aimpoint and Trijicon optics. My choice has been Aimpoint T1 on our 10" 516 SBR and Trijicon's MRO on our MCX and VCOG on our 14.5" 516...
Stick with the 400 for future parts availability and more customization options. HD use go for a good red dot, Aimpoint and Vortex are quality, have not heard or seen enough of Sig optics to put them in that category yet, a few years back Sig's optics were miserable but this is a new line now. I have not had an issue with gas face using a suppressor, adjustable gas block seems to keep it to a minimum. The only piston gun I use is a 716 and with a suppressor it still gets pretty dirty. I actually get more failure free rounds before cleaning is needed from a 308 DI rifle with a suppressor than I do the 716.
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