Picking up two of the same weapon that cannot hold spare ammo (displayed as "-" in the HUD) will not say ammo has been picked up, instead the pickup text will only display the name of the weapon. This is demonstrated with the Bow in the .GIF.
This is also the case with the M60 and the Flamethrower.

Since the ammo pool does get refilled it is probably best to mention it in the pickup text also.

