I just got an idea for a gun

Name : CG GUN SERIES
Type : High tec weaponry
Developers : Duffla weapon inc

Story CG 188 A
Originally developed as a mining tool by the civilian tools department of Duffla cooperation
This gun is the mother of all the CG guns .
During the skrith invasion of the outer rim planets it was commandeerd by the local defense groups
It was originally used to demolish important and tactical roads and buildings so the enemy couldn't
take advantage of them.
After a soldier discoverd the lesser conviniant usage the company turned it into a weapon.

Desription The CG 188 A "AIRBLASTER" consists of a medium long barrel atached to a backpack with a "hose"
The backpack is actually a huge compressor that compresses the air in a build in tank .
Range : medium
primary fire : fast firing small amount of compressed air that does little damage to the enemy but it does knocks back the enemy some distance every shot uses 1 power cell (disoriantating the enemy like the HVC )

secondary fire : fast stream of compressed air doiing no actual damage to the target but it's
capable to knock or push the target extremly fast away . every shot uses 2 power cells due to the compressor beiing on the whole firiing period. ( doiing damage if the target hits something ,the faster the higher the damage.)

Special fire mode : relaeses full presure in the tank resulting in a medium to high damage ( depending on the ammo left in the tank) expanding conicly shaped shockwave from the barrel pusing everything away. Including the user (moves backwards)

Ammo : Power cells to power the comppressor ( the more power the more the tank is filled with compressed air) about 10 units in each pickup ( fully powerd compressor takes 90 units of power cells to fill up the complete tank)

I think I like the idea for this. To be honest I had thoughts about something like this for a good long time.

Few suggestions though:

This really shouldn't be considered a weapon, story-wise. The company really should be a mining equipment company or something rather than an actual weapon company.

Now, to expand on your idea.

This alone, would be pretty useless for the most part I'd say.

But, this could be part of maybe other stuff. This could be something like a power system for several other mining tools. What you propose right there could be the base system with no tools, while other alternatives could be maybe a rock saw, a pneumatic hammer, and maybe something like a spike gun that just sends metal rods flying at a relatively low velocity.

They could all use the same ammo and have the air tank show on all weapons that are pneumatic.

As a stand alone weapon it is indeed a somewhat underpowerd weapon but it is more meant to be a tactical weapon used in small group tactics.
Defending the group and slowing down the enemy somewhat.

On the side the power system could perhaps be implemented ( maybe special fire mode to switch tools / weapons? )

Sounds cool to me.

you need a dynamite weapon, or even better, a tactical nuclear explosive...

Dynamite... that could make sense... give the above mentioned spike gun idea an explosive charge. The fuse could initialize upon hitting something, then maybe give a 5 second delay and it explodes.
This could actually make sense for a mining tool and could give people that sticky explosive they always want. Chances are though, the spike itself would kill the person.