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.