How would you interpret ‘four hundred twenty’ and ‘minnenwerfer’ in the second line.
1 How would you interpret ‘four hundred twenty’?
Does it refer to the caliber of the guns?
2 Do you know the meaning of ‘minnenwerfer’?
Is it some kind of gun?
Thanks. It’s from A Farewell to Arms by Ernest Hemingway (Chapter 9).

the context:
Something landed outside that shook the earth.
"Four hundred twenty or minnenwerfer," Gavuzzi said.
"There aren't any four hundred twenties in the mountains," I said.
"They have big Skoda guns. I've seen the holes."
"Three hundred fives."
We went on eating. There was a cough, a noise like a railway engine starting and then an explosion that shook the earth again.
"This isn't a deep dugout," Passini said.
"That was a big trench mortar."
"Yes, sir."

Question

How would you interpret ‘four hundred twenty’ and ‘minnenwerfer’ in the second line.
1 How would you interpret ‘four hundred twenty’?
Does it refer to the caliber of the guns?
2 Do you know the meaning of ‘minnenwerfer’? 
Is it some kind of gun?
Thanks. It’s from A Farewell to Arms by Ernest Hemingway (Chapter 9).

the context:
Something landed outside that shook the earth.
"Four hundred twenty or minnenwerfer," Gavuzzi said.
"There aren't any four hundred twenties in the mountains," I said.
"They have big Skoda guns. I've seen the holes."
"Three hundred fives."
We went on eating. There was a cough, a noise like a railway engine starting and then an explosion that shook the earth again.
"This isn't a deep dugout," Passini said.
"That was a big trench mortar."
"Yes, sir."

Answer

You are correct.  These both reference types of artillery.  The "420" was probably "Big Bertha" while the minenwerfer was a trench mortar.

Here's a list of big guns ranked by type of projectile and listed by caliber.
https://en.wikipedia.org/wiki/List_of_the_largest_cannon_by_caliber

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