As the creator of the Taihō Scalable Mass Driver I just want to note for posterity that it will fling anything of the appropriate size and magnetism. If it isn’t something that would survive, that something comes out as plasma.
ADNR has a theoretical max ultimate tensile strength of 310,000 MPa and a tested compressive strength of 435,000 MPa
The Helium is compressed to 1000 atmospheres (I kind of had to guess, Ideal Gas Law had it at 800, and I found 2 real graphs, one with units and hydrogen, and one without units with both hydrogen and helium, relationship was about 1.2 the Ideal Gas Law PV=nRT) generating an outward force of 101.325 MPa
Acceleration would exert a compressive force along the axis of (assuming a barrel length of 20 inches = 50.8cm) 320^2/(2*0.508) = 100787.401 N or ~101 MPa in the case of the 10*50mm and 2000^2/(2*0.508) = 3937007.874 N or 3,937 MPa in the case of 15*75mm.
In the case of 10*50mm the force of acceleration and the force of acceleration and the force of pressure are actually in opposition and so would almost perfectly cancel each other out, as far as material stress is concerned anyway.
Without doing full CAD designs and simulations on the projectile I'm comfortable to say that the idea of it surviving acceleration while having the walls thin enough to carry the payload described is plausible enough for science fiction.
