So I went to the range this morning and my AR, which has been 100% reliable to date, abruptly quit cycling.
I fired a few shots and then had one round get hung up half-fed into the chamber. Resumed shooting, another couple rounds and the bolt failed to lock open on the last round. Changed mags, chambered the first round and fired, ejection pattern seemed different and bolt failed to pick up the next round, skidding over the top of it instead, leaving a scrape on the brass.
This pattern of chambering the first round, firing, different ejection pattern, and the bolt riding over the next round in the mag and not feeding continued until I gave up. Four different mags and two types of ammo were used all with the same results.
The gun seemed to be short-cycling. I took it apart and noticed nothing amiss, as best I can tell the gas key is unobstructed.

Rifle: BCM midlength upper on a lower with a rifle buffer and spring. Gun has been 100% until today when it has a complete meltdown.

Ammo: Privi M193 and Radway Green SS109, gun has had no issues with either type, and several others, including Wolf, in the past.

Magazines: two Magpul 20-rounders, a Magpul 10-rounder, and an Okay GI 30. None of these mags have ever had issues in this gun or in any others.

Lube: Slip EWL30. My usual lubricant. Gun was amply lubricated.

Ejection: normally this gun ejects at around 4 O'clock or so, I did notice that it seemed to be throwing them out at closer to 3 O'clock and not quite as far as usual. I didn't examine the brass as it landed in the snow but in hindsight I wish I'd dug some of it up.

Environmental factors: approximately 30 degrees. I add this because I'm wondering if perhaps the thicker Slip EWL may have been gumming up the gun somehow in the cold weather. EWL is my regular lube but this is my first time shooting this gun in weather this cold. Although a week ago I shot the gun in only slightly warmer 33 degree weather, with the same ammo and mags, without issue.

Any ideas?