The enemy sees it as well, but not confirmed. I asked them to watch it before I shoot and I counted down. At least 2 of them confirmed they see when I'm holding it to shoot. They were'nt that troll type players, we played some rounds against each other.
But still, it is not proven, but it seems that the m appears to everyone, not just teammates...
@Lestath81HUN I did the same thing and they said they didn't see anything. Maybe they thought that you were asking in general if they ever saw it, or otherwise misunderstood? Or, the people I asked may have been mistaken. It would be easier to find out if you're on the receiving end of the RMN 50 nade (if an "m" appears right before you die from it, or if an enemy shoots it and you see if the "m" appears).
