Mr. Nagant was a 'Walloon', from the French-speaking part of Belgium.
The pronunciation is ka-BOOOOOM!!
Saw a photo of a Wehrmacht soldier carrying a 'damn commie gun', on foot on the Russian front, escorted by a small Panzer (tank).
He had a semi-auto Tokarev in his hands. They might not have had the Sturmgewehr at that time.
Did you ever notice in the movie, Kelly's Heroes, that the sniper rifle used by the G.I. in the bell tower was a Mosin? Considering, the scene took place on the Western front, I have always wondered about that.
Sergei Ivanovich Mosin was Russian. Léon Nagant was Belgian.
MO-seen nah-GON (with the "GON" heavily nasal).
Of course, in the United States, you will most commonly hear it simply called "MO-zin", or "MO-zin na-GONT". Not exactly correct, but resonably close enough as an anglicized version of the original.
P. S. - That's a friend of mine shooting my 1944 carbine. They're KNOWN for that dramatic muzzle flash.