I think everyone here is missing the point that the reduction in reserves restricts the usage of the gun to be solely for boss DPS. Before you had some leeway as to whether you could expend some ammo on anything else besides a high HP target that will be able to survive at least 3 shots from the whisper. So basically to you know, use the gun as a sniper.
Now if you decide to use the gun that way, if your luck with heavy ammo bricks falls short you end up with less power ammo than if you were to use a different weapon in a similar role. As a little hitch, yes the gun also becomes less forgiving with mistakes especially when white nail doesn't trigger properly.
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You're spot on. Effectively they're treating it like every other heavy weapon which is how it should be. You can't just run around shooting anything with it and still be guaranteed to have the heavy reserves you need for the boss. It's technically still more forgiving than any other heavy because if you're at least semi-skilled, and the game doesn't get too buggy with the perk activating, you can theoretically use it fairly often and still have ammo for the boss. There needs to be some risk/reward with heavy weapons and the reserves being reduced put it in its proper place for a heavy.