biggest shotgun used in the war (actually of all time?)

I was going to say 37mm canister rounds, but I remember 57mm canister shot was put to use in the last year or so of the war.

__________________
All the Best
Jack

Well, the Germans and US had a 75mm cannister round too. But, these are far smaller than historical ones used in the 19th century that went in smoothbore as large as 11 or 13" in the case of naval guns of the period.
As for small arms, the largest manufactured shotguns were 6 gage or 1 bore (that is, 1" in bore). These were often used for big game hunting like the famous Holland Nitro Express guns.

__________________
Truth is stranger than bullshit!

T.A. you're right, I was stuck on 10 or 12ga for a bit. "Used" during the war narrowed the question, well....memory fades again. Carry on men.

__________________
All the Best
Jack

ok, the answer is 18.1-inch bore. the san shiki anti-aircraft cannister shell for the yamato's main guns (designated #6 shot if it remember.) the shot load varied, from several hundred 1-inch diameter steel balls to steel balls with a 50-gram explosive charge (about the strength of a 20mm cannon shell.)

wasn't very effective though.

The 3 Shikishan Shell is not a "shotgun shell." Its closest equivalent was a case or shrapnel round. It did not come apart at the muzzle of the gun. Instead, it was fired to altitude using a time fuze that initiated a burster charge and then scattered the individual incendiaries ahead of it just like other case rounds did. It did not have a filler of "steel balls with a 50 gram explosive charge."

Here's a thread explaining the round in detail:

Sanshiki shells?

__________________
Truth is stranger than bullshit!

hmmmm.... yap.