You can use animated weapons with the USK.
To add animations with animated weapons do the following:
Import an empty animation with all weapons in the root (for example, T-Pose animation).
Create the Avatar Mask with this animation avatar.
Make sure that all animated weapons and their parts are selected in the mask
Add the mask to weapons animations. In each animation which contains an animated weapon, you must add the mask which you created before
Adjust the main Masks (Tools -> Assets -> _Masks -> BodyTop (tp,td) & BodyTop (fp)) with the T-Pose avatar (which you've imported in the 1st step). Select in the masks only animated weapons parts.
To learn how to synchronize animations with the player’s actions (for example, the spawn of magazines when reloading), read this page: