get both health and ammo from packs
+20% damage ramp up

-66% health and ammo from packs
+20% damage fall off

you can't actually reload it because there's a metal piece where you're supposed to insert the shells, so i think that it would more like a beggar's bazooka/widowmaker:
+hold fire button to prime the gun, in which shells would be "loaded" like that of the beggars bazooka, with a max of 4 shells. Release fire button to unleash each shot with a +70% firing speed.
+ammo is used with metal; 20 for each shot.
+80% of damage is returned as metal.
-20% damage
-20% accuracy

So it would just basically be the original version of the panic attack?

I'm thinking by its design it could be another battle engi weapon, perhaps at a longer range and with only 2 shot, but those 2 shots really pack a punch for picking off single targets. Could be paired with the wrangler or just pistol to be more of a mid-range threat as engi.

Not affected by damage falloff (shotgun spread could still be a limiting factor for distance though)
+25% tighter bullet spread
-25% slower reload speed
-66% clip size (2 bullets in chamber)
+40% damage bonus (point blank will be 126 damage)

Maybe if you're feeling adventurous, a meter of some sort that can be filled? Maybe it allows you to deploy a second mini sentry for every time it is filled, but no more than 2. As to whether or not it would need better balancing, you'd need to test it out in game honestly. It sounds decent to me.

It looks like something what Junker Queen would use. Maybe something with healing the Engineer? Sentry damage heals the wearer? Picked ammo packs dropped by enemies killed with this weapon heal the wearer?

+ Instead of a spread, this weapon fires a single slug that deals increased damage.
+ 50% damage bonus
+ This weapon has less damage fall-off

- 33% clip size (4 rounds per full reload.)
- Wearer cannot gain metal from their own dispenser.

Alternate Idea

+ Deal damage with this weapon to automatically repair your sentry gun.
+ This weapon can damage sappers
+ Damage done to enemy buildings + sappers will be returned as metal

- 33 clip size (4 rounds per full reload.)
- 15% blast damage resistance

Originally posted by Dr. Martex:

+ Deal damage with this weapon to automatically repair your sentry gun.

The Rescue Ranger was already nerfed because of infinite ammo reserve.

Originally posted by Dr. Martex:

+ This weapon can damage sappers

Why make the Spy even worse?

Originally posted by Dr. Martex:

+ Damage done to enemy buildings + sappers will be returned as metal

Worse the Widowmaker.

-10% sentry damage
+ammo from building scraps
gives bleeding effect to anyone hit

= Consumes 8 metal per shot and does not need to reload.

+30% fire rate.
+ Shooting at an ammo or health pack converts it into a Combo Pack.
+ On Hit: Heals your most damaged building.
+ Damage breaks sappers.

-40% pellets per shot.
-20% metal from pick-ups.

Hitting a med kit or ammo box turns them into a combo pack that contains both health and ammo. Contents are of the same size as the original pick-up, be it small (20%), medium (50%) or large (100%).
Damage dealt with this weapon heals the building with least health with a 2:1 damage-to-healing ratio. A visual indicator line connects the user and healed building.