It sounds like you might be manually updating the gun transform on the tick, instead, have you tried parenting the gun to a Motion Controller component on the player pawn? That should achieve the snap you want.
For alignment or any offsets needed, you can try loading one of the Vive controllers into UE4 and temporarily add it to your weapon Actor to align the gun model. If you have SteamVR installed, the Vive models are within the same directory …\Steam\SteamApps\common\OpenVR\resources\rendermodels