It is possible you downloaded a weapon model which was designed for an older version of the game, and now it's completely broken and incompatible.

go ingame and type the command "/hudgunsdir "name of the folder with your new hudguns"" e.g. /hudgunsdir new_hudguns, after you placed them into sauerbraten/packages/models

It would help if you say which hudgun models you've downloaded and where you've put them.