What was that big gun?

It was a railroad gun.

And a really big one at that.

Or rather, they were really big ones.  There were several.

Details on the giant long range guns are surprisingly sketchy.  They, or it, or whatever, never fell into Allied hands and the Germans took what was left of the guns, whatever that was (and it was likely most of it, or them), back into Germany as they retreated towards the end of the war.  What they couldn't take, they destroyed.

The guns were apparently 211mm guns, a little over 8in.  That would remain a large gun today, but not so large as to not be deployable then or now as field artillery. The M110 Self  Propelled Howitzer, for example, featured an 8" gun.

 

 M110 Self Propelled howitzer

The Paris guns weren't howitzers, however.  They were rifles.  Extremely large rifles with very long barrels.  And over time, likely due to barrel wear, they were bored out to 236mm, 9in.

 They were transported by rail, and indeed they featured a rail turntable as part of their emplacement.  

And they were weapons of terror.

Say what you like about the Germans prior to World War Two, but at the end of the day, the fact of the matter is that the German military seems to have been uniquely prone to acts of terrorism as early as the Franco Prussian War.  And during World War One they undertook several strategies and weapons that were frankly terroristic, of which the giant railway guns were merely one example.  Their only purpose was to silently shell Paris at extreme long range, some 75 miles, and with accuracy that was no better than to simply hit the city, which the guns did at least twenty-one times on March 21, the first day they were used.  They kept up that rate of fire for a considerable time thereafter until the end of the Kaiserschlacht in August and the ultimate reversal of German fortunes mandated their dismantling and removal.  They were never captured by the Allies.  It's telling that while the Versailles Treaty required them to be turned over to the Allies, the Germans did not do it.

250 Parisians were killed by the giant guns and another 620 were wounded.  On this day, Good Friday, 1918, a projectile from one of the giant guns went through the roof of St-Gervais-et-St-Protais Church, killing 91 and wounding 68.

The aftermath of the March 29, terror shelling.

There is no excuse for their use.  There wasn't then, and there still isn't.

Super artillery, into which these guns class these fit, went on to see some use during World War Two by the Germans again, but the advent of aircraft meant that they had become too vulnerable for much use, although they never saw all that much use to start with.  The Germans would deploy some super artillery in the East during World War Two, but the manpower required was so vast that the use of the guns has been calculated to be a net detriment to the German war effort during World War Two.  They didn't achieve much during World Ware One either, as the Parisians grew blase about the big guns which, as destructive as they were, were unlikely to actually get any one person in a city of millions.  In modern times super long range artillery has not seen use although it has been studied with at least Baathist Iraq having taken an interest in them, and having studied a gun that would have been capable of hitting Israel while being fired from Iraq.

Seems if a nation uses these its cause is dishonorable by definition.