Here's my idea:
When you fire it, it does a typical shotgun blast but it also causes the umbrella to open, limiting visiblity, but granting one or more of these effects for as long as you hold the primary fire button:
Knockback resistance
A much weaker base jumper float
Immunity against splash-based attacks(Jarate, mad milk) to you and allies hiding behind you.
Immunity or resistance against defuffs to self and teamates behind umbrella
Expolsives explode farther away from you beacause they hit the umbrealla first

Once you realease the primary fire button, the soilder pulls the umbrella closed. Once the closing animation finishes, the umbrella/gun can be fired again(Closing an umbrella is like pumping a shotgun), reloaded, or switched out.
I suggest having a clip size of two to four.