That would depend on your definition of "biggest" gun that I've ever fired.
If you're talking about shoulder-fired or hand-fired weapons the largest barrel id would be a 10-gauge, 3-1/2" magnum shotgun. Handgun would be either a .45 ACP semi-auto (SIG Sauer P220) or a .445 SuperMag revolver (Dan Wesson). The .445 is a LOT more powerful than the .45 ACP, an old favorite of pin and silhouette shooters.
Some of the more unusual guns would be a bolt-action, single-shot .22-caliber pistol chambered in .221 Remington Fireball: Remington XP-100, mid '60's vintage, weapon was available later on in a number of other calibers that were also traditionally considered to be "rifle" cartridges including .308 Winchester (almost exactly, but not quite the same as the 7.62 NATO round (used in "battlefield", as opposed to "assault" rifles), although the two calibers are apparently interchangeable. A VERY accurate, and deadly "varmint" pistol. A number of Kennedy assassination conspiracy "theorists" speculate that the "second gunman" was using one of these in Dallas too.
Then there was that .22 caliber rifle chambered for a Wildcat cartridge, the .220 Swift. That bullet left the barrel at almost 5,000-fps, ACCURATE range of almost a mile with that 30-power scope. GREAT for varmints, bullet would literally explode after penetration (a LOT of hydrostatic shock caused when the "target" slowed that bullet down that quickly). Just don't hit a twig or blade of grass . . .
Also have fired a couple of single-shot, break-action 68-gauge shotgun pistols. (.410 caliber shotgun for those unfamiliar with that discrepancy in the shotgun sizing system). If I remember correctly one of the two was an over/under that sported an upper rifle barrel chambered for .22-caliber long-rifle.
That's a bit of an exaggeration with the blade of grass (although any unintended contact with the bullet along its trajectory path will cause deflection.) But the .220 Swift's bullet is comparatively light at around 55-grains (about 3.6 grams) and has a low cross-sectional density, depends almost entirely on its velocity to deliver its "punch" accurately to the target. It also depends on its "spin" to remain on a true trajectory between the gun and the target. Give those two conditions its VERY easy to deflect it off course. Think about a passed football that's touched by a player mid-flight. It doesn't end up anywhere near where the quarterback wants it to go. There's actually a bit more to it than that but that should give you the general idea.
Oh I see, I see. Sorry, I missed the subtlety of the comment. I understand. That's clever. ) Well, Salt, your answer is pretty magnificent in its scope and knowledge. I couldn't pretend to be familiar with all the weapons and ammo you have cited. It is always good to come across someone who knows what they're talking about. Thank you for taking the time to write.