I second 'Ranger Z.' You would be doing a variation of what we refer to as sporterizing- permanently altering a weapon from it's original configuration. That's been described as a good way to turn a $500 "collectible gun" into a $100 "used gun." Not a bright idea.
Another problem with altering original guns is the potential for creating a "fake." Please understand. --- I am not accusing you of faking anything.--- But we only get to possess these for a little while before they will pass on to another in their own time. Original 'trench guns' are high dollar items. It would be very easy, especially if ignorant of your modifications, for another to later, (knowingly or not) represent your gun as an original 'trench broom.' It has happened before.
But you can have your trench gun and enjoy it too. Again referring to "Ranger Z," there are Chinese copies of the model 1897 with, I believe, 20" bbl. You can modify those any way you darn well please and may even, within reason, get a return on what you put into it.
"In times of trouble and of war, God and soldiers we adore;
When wars are done and wrongs are righted, God's forgotten and the soldier slighted." Olde English proverb, origin unknown
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