Combination Weapon - Pnumatic Coil Heavy Rifle

So for late game I was thinking of an upgraded coil rifle for Mid-Late to Late game.

Essentially uses compressed air for the initial speed gain for the projectile, then an extended upscaled set of Magnetic coils in a longer barrel to accelerate the projectile further for increased damage and armor penetration.

Using a few Betavoltaic Cells to power a small compression pump and to off-set the higher draw of the larger set of magnetic coils could keep the Kj draw reasonable (from 2 up to 4-5?).
A built in battery/capacitor stores excess energy from the Betavoltaic cells when the compressor is finished filling the built in air tank to reduce power draw from UPS to maintain reasonable consumption.

The barrel would be 50%-100% longer, it’s weight would nearly be doubled and encumbrance when worn would be pretty big.
But the rifle would be more powerful, longer range, still very quiet, and could penetrate light armor with a cheap and abundant ammo.

The recipe could be along the lines of : a Coil-Gun, a 2 Liter Metal/Polymer Tank, a compression pump, a small/micro-electric motor, 8-16 Betavoltaic cells, A Medium/Large battery OR a Small/ Medium Storage battery, A UPS, 30 electric Scrap, 150 Copper wire, 10 polycarbonate sheets, 10 plastic chunks, 3 plastic gaskets and 2 rubber hoses.

Or maybe a different way of acquiring, maybe using equipment in the labs?
I think it would be neat if there were 3-D printers in the labs that if provided power could be used with Blueprints on USBs to produce extremely high quality devices? (I’m gonna post a separate post for that idea)

Also as a side note: Could we get something like white phosphorus coating for flechette? For dealing with those who have that pesky regeneration ability.