[ThunderCactus]

FYI you'll also need enough current to have that piston generate up to 80psi

And keep in mind, the piston body doesn't need to be there, only the piston head. So you could just use a long chain of solenoids with a rod that has the piston head on the front.
If you run out of room in the mechbox, remember you can always extend back into the buffer tube.

Multiple segmented solenoid with a segmented rod in order to up the pressure. Don't know how well that will work given then length of the stroke though.

Stroke will shorten as the barrel gets shorter: shorter barrel = shorter cylinder
With electrical power you should be able to accelerate fast enough in a very short distance, and all the power to generate initial pressure is going to be applied electrically instead of by momentum.
So you're going to need some capacitors lol