I have one at my shop for $1,099. It is an M4 profile barrel w/ carbine gas length. I have never heard of the 6400. Thoughts?

You can not go wrong with a Colt.
6400 is just another series.  Look at their web site.

the MT6400C is the ban compliant M4, it's basically a 6920 with a non collapsible M4 stock, smooth muzzle with roll pinned comp, shaved bayonet lug, and goofy colt 9rd magazine.

I have one that is now an SBR.

Possible to do at least that well on a post-ban without the neutering (CDNN has a Colt M4gery for $1099), but I see you are in NY.  It probably has the half-circle BCG (can you replace that with a full-circle M-16 carrier without doing time up there?); the extra mass is useful, and the shrouded firing pin even more so.  
It is also muzzle heavy as hell, so you might want to send it off to ADCO for a little Jenny Craig treatment.
And the barrel might not be chrome lined.
I had the 20" version of the same rifle, and can't say I was really unhappy with it; just moved it on for other things I liked better.  The prancing pony does count for something.
Moon

the MT6400C uses the same government 16" 1/7 twist 5.56 chambered chromelined barrel as the LE6920 just without the threaded muzzle. M4 feedramps, M4 cuts in the upper, F height FSB, painted T marks, H buffer, side sling swivel, carbine extractor spring and insert.

I had one I bought during the ban. I even had it "de-banned" after the ban ended.  Had the pinned stock replaced with a collapsible one...had the barrel cut to 14.5 and perm attached an extended A2.  Ran it through a 3-day 1000+ round Randy Cain carbine course with zero malfunctions or problems.  Just never could get past the fact that it originally started out as a bastard ban compliant gun.  Sold it and bought a Colt 6921 Registered SBR.  6920 is the same thing but with a 16" barrel.