I heard from some that Colt AR15's aren't mil spec. And are they high quality?
Until a few years ago, Colt ARs were quite a bit different than "Mil-Spec". They used hammer and trigger pins that were larger than the Mil-Spec (.154"). These are referred to as, "Large Pin Colts." They were top quality, but you had to buy specific trigger parts.
At one time, Colt put an uber-ugly sear block in their lower to prevent folks from installing a drop in auto sear (DIAS).
Bolt carriers had the lower-rear cut off completely, so that only the upper half would contact the buffer (no tripping of auto sears or Lightning Links).
At one time, Colt also used a hinge pin whose center was different than Mil-Spec, requiring the use of offset pins to use a standard upper on that lower.
None of these things were required by law or regulation. Colt was doing it for their own reasons. During this same time, Colt wouldn't sell the cool guns (CAR-15/Lite/etc) to the general public and this began well before the 1994 Assault Weapons ban.
Having said that, these were well-made, but you did have to keep the differences in mind when you bought parts. Somewhat irritating.
Since about 2009, Colt has had a change of heart and has switched to the Mil-Spec pin diameters for all guns, auto bolt carriers, cool configurations, etc. The quality is still top-notch, but they're selling their whole line to the general public.
A newer 6920 should be as mil-spec as it gets.