score:2
You are ignoring the very real differences between gun types and their ordnance.
Direct fire:
These weapons fire along line of sight at high speed and at very low elevations. They include muskets, rifles, cannons, and - in a later era - anti-tank weapons. Much of the damage is done by the velocity of the projectile, whether a solid shot such as a cannon ball or bullet or an exploding one like an anti-tank ordnance. In order to obtain the high muzzle velocity a high ratio of charge weight to projectile weight is required - consequently also improving accuracy especially once rifled barrels became common place. The projectile approaches it's target at a nearly horizontal elevation
Indirect Fire - Howitzers:
These indirect fire weapons fire at elevations of from roughly 15 degrees to 45 degrees. The increased angle of fire allows for a much lower ratio of charge weight to projectile weight, allowing for heavier projectiles - ideal for high explosive ordnance where the bigger the bang the better. Accuracy is less than for an direct fire weapon, so effectiveness is only obtained for larger targets.
Indirect Fire - Mortars: These indirect fire weapons fore at elevations of from 45 degrees to 85 degrees. Some damage is done by vertical penetration of the projectile in addition to its explosive charge. Accuracy is reduced again relative to howitzers due to the longer flight time.
Note that the key difference between the use of howitzers and mortars is that howitzers increase range by increasing elevation (towards 45 degrees) while mortars increase range by reducing elevation (towards 45 degrees.
An effective anti-tank weapon (and its ordnance) must have the following characteristics:
High muzzle velocity to penetrate the armour (up to a foot thick on the front of heavy tanks) and allow acquiring fast moving targets;
Explode after initial target contact; and
Direct fire so as to be aimed at a smallish target.
An effective anti-personnel weapon and its ordnance must in contrast have these characteristics:
Medium muzzle velocity as formed units (and trenches) are fairly large and stationary or slow moving;
Explode on - or ideally shortly before - target contact; and
Indirect fire for effectiveness against targets behind terrain.
An effective siege gun
Muzzle velocity irrelevant as fired only at stationary targets such as buildings;
Explode on - or after as in a bunker buster - contact; and
Indirect fire for effectiveness against targets behind terrain.
Weapons commonly used in WW1 were designed for use against structures and personnel, so of the latter two types. When tanks appeared they could get by with quite light armour because no direct fire anti-tank weapons existed on the battlefield.
The rifles (calibre too small to penetrate armour), howitzers and siege guns (both too inaccurate and with too slow a projectile for targeting tanks effectively) of World War One were designed for a very different purpose than weapons designed to target and destroy small, fast-moving, armoured targets such as tanks. Other than lucky hits on the tread, there was little most WW1 units could do against tanks until specialized anti-tank weapons had been developed and distributed.
Upvote:1
A direct hit from a heavy howitzer or cannon (6" and up) against anything other than the frontal armor of a MBT should do serious damage. (I'll leave it as vague as that because I have no exact source handy. But WWI artillery shells were in the same ballpark as WWII artillery shells, and the frontal armor of late WWII tanks was better than the side, top, or rear armor of modern tanks. Big artillery could kill tanks during WWII if they could hit.) But as Pieter pointed out, these weapons were not designed for direct fire against agile ground targets.
Heavy coastal artillery and ships firing inland would also be effective and it might have a slightly better chance to track the tank, as long as they can bear at all -- coastal batteries were usually pointed seawards.
Upvote:4
The largest WW1 artillery pieces (especially the siege mortars) were very large indeed and even a dud shell from one would do serious damage if it landed on the engine deck of a modern (or indeed almost any) vehicle by sheer kinetic impact force alone.
For example the Austrian "Schlanke Emma" fired a 385kg projectile (yes...). It'd probably crush through the engine covers and do serious damage to the engine of a modern tank (or indeed any tank in history). Of course the chances of scoring such a direct hit would be miniscule, and even a near miss would likely do little but rattle the tank and maybe stun the crew for a few minutes.
Similar with the Paris gun used by the Germans. Smaller projectile but higher impact speed giving it more kinetic energy.
Then there was the massive Big Bertha firing shells nearly 900kg in weight. The impact of a shell from one of those would be similar to being hit by a 1000lb aircraft bomb...
And while the central powers had the most famous big guns, the British had several 12 inch howitzers and railway guns that would have made a big bang as well if a shell of one hit an armoured vehicle on the engine deck.
But none of these weapons were even close to being accurate enough or fast enough to aim and fire to be useful as anti-tank weapons.