Which M400 configuration?
BCG:
BCM
SOLGW
Sionics
LPK:
Colt through G&R tactical
ALG/Geissele
Sionics
FCG:
Utility type FCG with a nicer definitive "wall" and crisp break.
Geissele SSA
G2S (Same as SSA sans some QC checks, lack of J spring -> uses an external type, and laser etching)
SD-C (flat trigger bow)
Precision, lighter pull and generally "faster" feeling.
SSA-E
SD-E (same as SSAE but with flat trigger bow)
Receiver Extension:
LMT
Stock:
B5 Enhanced SOPMOD
Money no restriction, LMT SOPMOD
Take down pins:
Battle Arms Development EPS
Grip:
To each their own:
B5, MIAD, K2, etc....
Buffer and spring:
H2 BCM
SpringCo red spring
Charging Handles:
BCM Mod4
Geissele SCH or Airborne.
I like the Mod4 as its pretty much the prefect size to manipulate and clear an optic, but not so large as to snag. The airborn sized Geissele is a little closer to the Mod 4 latch size, but given Geissele latch shape, I prefer the std Super Charging Handle. YMMV....
That’s a list of solid, quality parts.
Like he mentions, some parts are personal preference. For instance, my favorite buttstock for a general purpose carbine is the Magpul MOE. It’s one of the cheapest ones, doesn’t have storage compartments or a coffee grinder, but does everything I need it to do; lock in place and give me someplace to put my shoulder and cheek.
The Vltor A5 is a very good buffer system. I have it in a few of mine. Includes receiver extension, buffer, buffer spring, and receiver end plate.
I like the Radian Raptor ambidextrous charging handle and the Talon ambidextrous safety/selector. The Talon can be set for 45 or 90 degrees. I used to think it was gimmicky, but now I really like the 45 degree option and use it in every single one of my AR’s, regardless of intended purpose or caliber.
A few other parts you can definitely do without, but that I’ve been using recently and really digging, are:
1. Geissele Maritime bolt latch. Quicker/easier to find to release or lock the bolt to the rear, nice for reloads and malfunction clearances. I’m a righty, but I especially recommend it for lefties.
2. Forward Controls Design Low Drag Forward Assist, if you’re running an ambidextrous charging handle.......if you’re not, then skip this part. It dispenses with the enlarged rim that is around the circumference of the standard forward assist. I’ve seen folks rip the crap out of their fingernail by catching it on this rim when using the right-side arm of the ambidextrous charging handle. The FCD LDFA fixes this nicely.........or you can take a file/Drexel and grind the rim off of your FA.
3. Forward Controls Design Enhanced Magazine Release. Every so slightly extended, with the additional of a cupped shape.