The real issue though is not whether these units had the capability but whether operationally they used it. AFAIK they rarely used it and for good reasons.
The ammunition was available in the theater. During the Battle of the Bulge, several HQs, smaller depots, and arty positions were overrun by the Germans. If there was ordnance with AT capabilities, attempts to put them to good use for self defense were made, most likely.
First, an it's a biggy, their prime role was not AT.
Of course, they were field artillery pieces, and not dual (or even triple) purpose guns (like say the German Flak 88's with their AA, artillery and AT roles).
Second, they were extremely vulnerable to direct or indirect fire once targeted.
Correct, they didn't even have shields for crew protection, so it would have been suicidal to employ these in a dedicated AT role.
With the tracked version, the M12 gun carriage, the thickest armor section (glacis?) featured 50 mm of armor, only, while other sections had only 12.7 mm of armor, barely enough to protect against regular rifle rounds. There was a chance that standard rifle AP rounds (used by the Germans) could pierce through some of those weak sections.
Its profile (with lowered gun) wasn't as tall as say the profile of a Sherman tank, though:
M12 in France:
While the tracked versions (which were not that prolific)...
Actually, they were
very successful during the assault on the Siegfried Line: The M12 gun motor carriages were brought foward to crack the Line's bunkers with direct fire, and were then dubbed "doorknockers" by US units. Since they could pierce through 2 meters of concrete, "bunker busters" would probably be the better term.
... could withdraw relatively quickly, the towed version took a long time to mount up and move. In that time the crew are fully exposed. IMO it would be a desperate or foolish commander that would use these guns in that role.
I am not talking about employing them as dedicated (dual purpose) AT guns, but about instances where (deployed) 155 mm-guns are threatened to be attacked or overrun. Call it "desperate" or "ballsy", but a commander in charge of such guns would have ordered to fire at incoming Germans. During the Battle of the Bulge, some units just got into such situation:
"The 333rd FA Group (333rd {
155mm},
969th {
155mm} and 771st {
4.5-inch} FA Battalions) initially supported the 2nd Infantry Division (United States) and its replacement, the
106th Infantry Division. At the onset of the
Battle of the Bulge they were eleven miles behind the front lines. With the rapid advance of the Germans, the 333rd FA Battalion was ordered to withdraw further west, but C and Service Batteries were ordered to stay behind to give covering fire to the
106th Division. On 17 December they were overrun with most killed or captured."
A Wikipedia quote (yeah yeah, i know), but I've read about these details somewhere else before (official Army history?). It looks like most of the pieces were evacuated (IIRC), but the 2 batteries were supposed to do both: Cover the 106th Division (where the bulk surrendered to the Germans on the 19th of December) and form a blocking position (direct fire), which didn't work out, apparently, as they got wiped out or captured.
According to what I've read, some of them put up a fight before they got killed, and since they had several small obs teams, they must have had time to traverse and aim, at least. I doubt that they just used their rifles/hands to fight the incoming Germans.