Title is pretty straightforward, it is bugged and I can’t transfer the M91/30 Mosin Sniper Rifle between campaigns, be it from Moscow to Berlin or from Berlin to Moscow.
Some rifles are the same in 2 campaigns but one of them has 1 star more (I mean base stars, no upgrades). And because of this star, they can’t be transfered.
Check if your rifles have the same ammount of stars. If no, then it’s not a bug, it’s a feature 
That is not the case, there are many weapons with different number of stars in different campaigns and you are able to transfer them, for example M1 Garand or Sten. When you transfer these weapons they just gives you more or less stars matching the star level for campaign.
In case of Mosin sniper rifles there is some difference between them that you won’t see at first, it’s the optic they use that differs in Moscow and Berlin, this is enough to count them as two different weapons unable to transfer. Same goes for Lee Enfield No.4 in Normandy and Tunisia, one has the bayonet and the other not.
EDIT: not a bug for reason above
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The amount of stars does not matter, it changes to fit the campaign you are transferring it to.
Its a bug.
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Not to a bug. The weapons use different scopes and are therefore treaded as different weapons.
As Volo said, the Lee Enfield Mk.4 can’t move from normandy as the Tunis one has a bayonet.
I forgot the one in Moscow had the PE scope, but it still does not make any sense, it has a removable scope so you should be able to transfer it without a scope.
Different star for same weapon in different battles.
Such feature looks like BUG.
Maybe with the smooth lining of weapon attachments it may be possible, but as of yet. Not quite.
The star system is irrelevant really. It just basically means later weapons cost more to improve.
If you move an MP40 from tunisia to Moscow or Berlin, it’ll just take the new star rating in the campaign.