Correct.
Shadowrun generally sets a general rule, and then spells out exceptions in individual cases as they appear.
So the general rule "weapon mods can go on weapons" is in effect unless countered by a specific exceptions like "this weapon cannot take underbarrel modifications", or "this mod cannot be put on X type of weapon".
Telling you what each and every mod CAN be mounted on would take up waaay too much text.
-k