How about a bag of ammonium nitrate fertilizer packed in the barrel?
They fire the gun, the shell detonates 10 pounds of explosive AHEAD of itself in the barrel.
Kerblooey!
Crap,
It occurs to me the fertilizer won’t be contained like the breach of the gun is. The explosion will just blow out the end of the barrel, followed closely by the shell.
You’d need something heavy in the barrel on top of the ammonium nitrate.
You’ve got a coaxial machine gun mounted with the main gun, though. The attacker could get sprayed off if they tried to get to the muzzle of main gun…assuming they could even reach it!
(Say, uh, does this thread remind anyone else of the old Saturday Night Live “Desert Storm press briefing” sketch? "I have time for two more questions. Yeah? " "Yes, Farud Hashami, Baghdad Times. Where are your troops, and can I go there and count them? ")
You’ll have to sort through this five part series to see which clip has it. I believe it starts at the very end and continues into the rest of the series.
I gallon of gasoline + 1 match
Damn near peed myself watching that sketch. 
Don’t recall the book, but they covered a Marine armored unit escaping the initial Chinese assault. The Chinese troops tried repeatedly to destroy by firing up the barrels of the tanks, in the hopes of either detonating the shells inside or getting lucky and getting rounds in when the breach was open. They weren’t successful.
Maybe if you had enough bananas(and cajones).
I’d start with a five inch gun and work up from there.
Declan
Where is the ‘gas cap’ on a tank? Could you sneak up and dump in a bunch of water or fine grit to clog the filters and/or damage the engine?
Could you “blind” the tank crew by spraying the tank with some kind of obscurant? If the periscopes/cameras/windows are all covered with ink, the tank crew can’t drive anywhere or respond to threats without opening a hatch, thus exposing their fleshy parts to enemy fire.
Man, this thread heavy on guesses, low on info.
Check out this Brief. I have no ability to fact check it.
That’s what we’re talking about- where does a gun (as far as tanks go) stop being “small arms” and starts being artillery? Maybe a .50 caliber only dings the armor; does a 20mm chip out pockmarks? Does a 25mm crater the armor or crack plates? The main gun on the A-10 Avenger (a.k.a. "Warthog) is 30mm, and that definitely is something tanks have to worry about.
The idea is that the tank is continually taking forlorn-hope shots from underarmed opponents and while no single shot has any real hope of taking the tank out, the tank is slowly getting beat up.
You can replace periscopes from inside the tank.
How many have they got? Or are you talking about retracting the 'scope to clean it, and then reinstalling it?
If they’re replacing them, then eventually they’ll run out.
If they’re cleaning them, then maybe an acid attack, or some kind of etchant that permanently damages the optics.
Either way, they would be at least temporarily blinded, presenting an opportunity for a more close-up attack that might be able to damage other systems.
I seem to remember that in OIF, a RPG round penetrated the side hull armor of an Abrams.
I suspect that from the sides and rear, M1s aren’t nearly as invulnerable as they seem to be frontally. I’d bet that many (most?) infantry AT weapons would penetrate if fired from those angles. They’d definitely penetrate the top of the tank.
Problem is, we generally don’t send tanks unsupported by infantry into areas where infantry could easily get shots on the sides and rear.
The problem is a tank isn’t uniformly protected. You’re not taking out an M1A2 Abrams from the front, as the armor there is over 2 ft thick. But a Bradley’s 25mm will take one out from the rear as the engine compartment isn’t as well protected. Your light weapons will go ‘ping ping ping’ from the front, and will take out the tank from the rear. A-10’s aren’t just hitting the sides and rear of tanks, they’re hitting the top as well, which makes them deadly. Because of the variation in armor thicknesses, your story goes from “we can’t touch it” to “well, it’s dead, now what” fairly quickly.
How much damage can the main armor matrix of an Abrams take before it begins failing? Who knows? The military won’t tell, so we have to guess on such stuff and frankly, there’s no data on which to make a guess.
It’s not intended to mock you… But the more I read the thread, the more I have the mental picture of a Tex Avery cartoon, with droopy putting his finger into the gun barrel, splashing the tank with a bucket of ink, etc… 
I freely confess that a lot of what I know about tanks comes from playing Battlezone some 30 years ago.
FWIW, the thread has shifted from “what’s the minimum weapon that will damage a tank” to “how can we stop a tank without anti-tank weapons” - at which point your only option is to get creative and hope for the best (while expecting to die).
The A-10 isn’t much of a threat to modern tanks. The early era tanks it was meant to destroy had critically weak top armor that the 30mm rounds woul penetrate, but subsequent generations were beefed up. It’s still useful as an anti-vehicle weapon - but not against modern tanks so much.
That’s the thing about armor - if you don’t penetrate it, you tend not to hurt it at all. You could ping a 20mm gun off the side of a tank all day and not hurt it. Something generally either penetrates or doesn’t, and if it doesn’t, no harm done. I don’t actually know what would happen in extreme cases (say a 75mm recoilless rifle being fired at one over and over) - I suppose if you could hit it in the same spot you’d eventually weaken the armor, but there’s not a lot of real world experience with this sort of thing since it’s a pretty implausible scenario.
How about luring the tank into attempting to cross a deep hole covered by camoflage netting? Would that trap a tank?