Don't worry about the gun.
Sea level pressure is nominally 1 bar. In a vacuum that would become 0 bar, so at a cruising altitude even if the hold is not pressurised (and it probably would be to equalise the load on the cabin floor) it will be a fraction of a bar.
Inside the tube the gun is good for close to 200 bar and maybe more. Another half a bar of differential makes no difference in practical terms.
The issue will be to do with airline regulations and pressurised containers.