I explained how I'd do it on the comments

Here's how I'd do it

The weapon would be a primary for Soldier that makes him move a bit faster as long as he loses some HP. Take 25 HP lost.

The base damage and accuracy will be double of those of the Stock SMG. Fire rate stays the same. Holds 30 rounds per clip with 90 rounds of reserve.

cool

Real

gun deals a flat 10 damage, it gets minicrit headshots, and can be used by any class.
30 magazine,
300 stored

300 reserve is just too good. 10 damage just makes it an SMG. Besides, that's the same damage the Syringe Gun has. So your concept rather fits as an SMG for Medic

forgot to add the long ass reload and the 30% accuracy loss

gun becomes gun

gun shoots someone

gun kills someone

gun