In article , Gary Coffman says...

The projectile in the cannon barrel can't exceed the speed of sound
in the barrel. With normal propellants at temperatures and pressures
tolerable by any reasonable launch vehicle, you could only get about
2700 MPH (about 4,000 fps). You could do a bit better with staged
charges (ala Gerald Bull's Supergun).

The germans were doing this during ww2 IIRC, with a long
barrel in the mountainside, and multiple staged charges
along its length.

Jim

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