M-18 may have a 76mm but it used a special type of AP round.
So did the 3" gun of the M-10 and the 76 in the M-4 since it was the same gun used in the M-18.
The ammo was so limited in supply due to the lack of tungsten used in its core that. in the 12th Army Group, only about one round per tank per month was issued.
About half the division's M4's were armed with the 76-mm. gun.14 With this gun, firing the new but scarce tungsten-carbide-cored HVAP ammunition, the tankers could penetrate the front belly plate of the Panther at 300 yards and at 200 yards had a sporting chance (about one to four) of penetrating the front slope plate. The division's tank destroyer battalion had also recently been equipped with the new M36 destroyers mounting the 90-mm. gun. And
RE: Pershing's in combat.
20 of the first 40 completed T26E3s arrived in Antwerp in January 1945 as part of the "Zebra Mission", sending new and experimental weapons into combat for evaluation.
All 20 were assigned to General Bradley's 12th Army Group and sent to the 1st Army where ten went to the 3rd Armored Division and ten to the 9th Armored Division.

In the last two weeks of April, the Pershings began to arrive in greater quantity. Third Army, for example, had ninety by the end of the month. On V-E Day there were 310 in the theater, of which about 200 had been issued to troops. But because of the difficulty of transporting them, and the time required to train crews in maintenance and operation, it is safe to say that the only Pershings that got into effective action were the 20 experimental models that First Army had received in February
THe first Pershing was lost on the night of February 25, ambushed at night by a Tiger I near Elsdorf. It was repaired and put back into action.
The next day a Pershing knocked out a Tiger 1 at 900 yards using a new T30E16 HVAP round.
On February 26th 1945 T26 number 38 was knocked out while guiding a roadblock. The position of the tank was poor as it was silhouetted by nearby fires and thus a conspicuous target. A Tiger I concealed by a nearby building fired 3 shots, the first of which penetrated through the coaxial machine gun port and whipped around inside the turret killing the gunner and loader. The second shot hit the muzzle brake of the 90mm gun causing a round chambered in the gun to go off which burst the barrel. The third shot glanced of the upper corner of the turret on the right side and took off the open had cover of the commanders cupola. The tank was repaired and back in action on March 7th in spite of limited spares availability. The Tiger got stuck trying to escape and was abandoned by its crew.
The next day T26 #40 knocked out a Tiger and 2 Panzer IV's at Elsdorf. 4 shots were fired at the Tiger at a range of approximately 900 yards, the first being a T30E16 HVAP which destroyed the tanks final drive sprocket on the left side. The second shot was a T33 which penetrated the bottom of the gun mantlet causing an explosion. Two other shots using HE were ineffective. The Panzer IV's were knocked out at 1,200 yards using one T33 round each with an additional round of HE fired at each to engage the escaping crews which caused considerable external damage to the vehicles.
In the fighting around Cologne on March 6th Pershing #26 famously destroyed a Panther in front of the cities cathedral with 3 rounds of T33 shot, an even caught on film and endlessly played on countless cable TV documentaries. #36 meanwhile knocked out a Tiger using 2 rounds of T33 shot.
Also in the Cologne fighting T26 #25 was knocked out at around 300 yards range by a Nashorn Panzerjager, whose Kwk 43 8.8cm gun was a much more effective weapon than the older Kwk 36 8.8cm used in the Tiger 1. A round, probably Pzgr.39 penetrated the lower hull front by the drivers feet and burned the turret of the tank out in an ammunition explosion. All of the crew escaped. This tank was repairable but instead stripped for spares and scrapped.
Another Pershing, #22 was hit by 2 high explosive shells from a 15cm field gun which caused extensive damage but that is a bit outside the scope of your question.
No King Tiger tanks were encountered by T26's so one can only speculate what might have happened.
Source(s):
Hunnicutt's "Pershing". Feist Publications
M-36:
Some assessment of the 90-mm. as an antitank gun was possible after the commitment of the 702d Tank Destroyer Battalion's M36 tank destroyers in the November Roer plain battles. The shell of the 90-mm. gun would ricochet off the 7-inch front armor plate of the Tiger tank at 3,000 to 3,500 yards; to be effective, the tank destroyers had either to get closer or attack the more vulnerable sides, and this fact the enemy evidently knew, for he had usually managed to keep his Tigers at a distance and expose only their heavily armored fronts.25 But to say that the 90-mm. would not defeat the frontal armor of the Tiger is not to condemn it as an antitank gun. The Tiger, cumbersome and underpowered for its great weight, was mainly valuable when the Germans were in a commanding position, as at Puffendorf, dug in on the defensive. Against the Panther, which most experts considered the Germans' best tank, the 90-mm. gun was far more effective than the 76-mm. In the tank battles on the Roer plain during November, the 67th Armored Regiment with three battalions of Shermans could claim only five Panthers; the 702d Tank Destroyer Battalion armed with 90-mm. guns claimed fifteen.
So essentially 20 Pershing's saw service and, possibly, combat. They certainly didn't see service in WW2 in "divisional" strength but tank destroyers, although there were alot more, were also scattered throughout armored divisons.
If you want to compare the numbers of Pershings to Whirblewinds or Ostwinds you need to take into account the unique niche that they serve in game as "armored" AAA versus just another tank with a big gun.
wrongway