We use a fair bit of Wyvern with LOUD, so if you could specify the unit in question, I can have a look at the rigging and see if we solved it. I'll be honest though, quite a bit of Wyvern was lifted from other mods, so we discarded significant portions of it.
If a second weapon is utilizing the same muzzle then this might explain your issue - the aim is being co-opted by the second weapon. Beams have no arc - whereas projectiles usually do, and the muzzle velocity, in conjunction with that arc, may in fact be raising the muzzle bone ever so slightly, throwing the aim off.
You can test this by disabling the 2nd weapon temporarily, to remove any such interaction.