My advice as a new M400 and rifle owner (new in September)....
S L O W
D O W N
A N D
G O
S L O W.
Don't make my mistake and jump into purchases prior to knowing what you REALLY want.
My example. Prior to buying the M400, I knew I wanted a red dot. Read/researched, and settled on the Lucid HD7 as mentioned above. HD7 is an outstanding red dot. But it is x1 magnification. Turns out for me, 50's yrs old, I can't use it past 25-50 yds. Soooo, why have it given our M400's are tack drivers at that distance with just the iron sights.
So, more reading/research. Settle on SigTac CD-4, get one for a great price. I've been running it for a few weeks. It is fixed x4 magnification. Thought I was ordering a 1-4x, my mistake. It is a fine x4 red dot.
Co-witness: HD7 works perfectly with iron sights co-witnessed right on the red dot. The CD-4, no. Rear flip down sights are fine set down, but my fixed front iron sight is in the way somewhat. Will need a mount to raise it if I keep it.
At the moment, I have no idea what I want. 1x, 4x, 1-4x scope, 3-9x scope.... I was at the range today in the rain (doesn't bother my MK25 one bit...) and 4x worked fine at 50, 75 and 100 yds. 100 was pushing it though (4 in 6 inch box, 7+ in 4 inch target box, 3 in 2 inch box).
I might the selling both the HD7 and CD-4 and getting a scope, we'll see.
I can't answer my own question: what do you want to do with the rifle? as yet.
I did see for the first time a video of a build where the red dot was on a mount that flips to the side. Interesting.