Mosin is the correct transliteration of the Russian
The correct transliteration from the Russia is "Mosin" and thats what I stick with, but I recognize the alternate spellings have been used for a long time before Brit actors played in "Enemy at the Gates".
Some of the comments I have had to post on Wikipedia Talk:Mosin Nagant:
NRA Guide to Firearms Assembly "Mosin's name is also encountered in arms literature as Mossin, Mouzin, Moisin, Mossine. etc., depending on the nationality of the writer."
In Roy Dunlap's Ordnance Went Up Front (Samworth, 1948) he refers to the "Moisin-Nagant" consistently; Dunlap served the length of WWII in the field with US Army Ordnance from North Africa to the Pacific. Of course, the official translation is still Mosin-Nagant; just noting that alternate spellings or transliterations can be found.
W. H. B. Smith "Rifles" (NRA and the Military Service Publishing Co, 1948) also lists the Russian Model 1891-30 as the "Moisin" so this alternate spelling was commonly used in 1948, not just an error or mistake by Roy Dunlap.