Springfieild M1903 with Grenade Launcher - Doesn't work - I am missing something?

I got a Springfield M 1903 with Grenade Launcher. I assigned to soldier, however, your supposed to use the “U” key to put on the grenade, I assume, but nothing happens, I don’t have the “U” option for the grenade attachment, Am I missing something or is this a bug, or am I just plain crazy and need to purchase something something or upgrade something in Upgrade section. I got it because I thought it was cool, or was it a waste of time and doesn’t work??

It’s a known issue with weapons with attachments: Gaijin.net // Issues

No disrespect but… Skill Issue, use real guns!