Okay, but the argument is weapons that are still around in some form, not the ammo.
The ammo is the weapon, and that's me extending a lifeline because flat out anti-tank guns aren't around anymore period.
They didn't just use the to disable tanks, though. The British Boys AT rifle was also used it against lightly fortified positions, armoured cars and other vehicles. It was replaced as an anti-tank by the wonderful and bizarre (if surprisingly effective) curio that was the PIAT, and for everything else by the M2 Browning.
Somewhat because it was already bad at it's job when it came out. It was approved in 1937 and by 1939 it couldn't reliably penetrate tank armor. It was designed as an anti-tank round (and gun), but all they made was an anti-material round (and gun). By this point in the war, traditional armor piercing techniques that would work against light armored vehicles bounced off of tanks, everyone that was still trying to make anti-tank rifles were using what amounted to shaped charges to make single use thermal lances. Even then, most anti-tank rifles didn't work that well, but German panzerfausts did, and so the whole idea of a man portable rifle for killing tanks died and they went to recoilless rifles and down that path.
But an anti-tank rifle is a rifle designed to disable or defeat tanks, there are even terms for what way you disable a tank. There are 0 man portable rifles capable of defeating contemporary tanks today, and nobody is trying. Note, an APC is not a tank, just saying there are rifles capable of defeating APCs doesn't mean there are anti-tank rifles.
Sure, but after that coversion it's not a rifle any more, it's an autocannon.
They well and truly just took an anti-tank rifle, swap out the fire control group, and put it on a mount to make an improvised AA gun as their military doctrine. But the Finnish, like the Russians, are somewhat excepted from things because a lot of the Finnish military was ad-hoc and made up mostly with what they could
steal borrow.