A few days ago I asked what was Mach One - the speed of sound - in cold 2 bar hydrogen gas. I found an "ideal" equation √[γRT/M] where γ = heat capacity which for H2 is 1.41, R is the ideal gas constant, T is the temperature in Kelvin, and M is H2's molecular mass. I didn't trust we'd be ideal. Lately I figured - waaaait a minute, somebody's probably done this for us already. So: AH Hodge, "An Experimental Determination of Ultrasonic Velocity in Several Gases at Pressures Between One and One Hundred Atmospheres", The Journal of Chemical Physics 5.12 (1937), 9747. doi:10.1063/1.1749973.
At 273.15 K, Hodge sketches a linear graph going up 8.5 "m/sec" for every 10 bar of pressure added. 1199.5 m/s at 1 bar, 1207.2 at 10 bar, 1215.7 at 20 bar (it looks less linear around 100 bar but whatevs). So 2 bar should be 1200.35 m/s. At 1 bar √[γRT/M] collapses to 72.577√T; at 2 bar, 72.629√T.
Not much difference; 72.6√T will do for the extent of the gunbarrel. As for T, Antarctica especially under ice in darkness would track T = 200 K. Speed of sound will be 1026.7 m/s. Assuming it's not diluted with oxygen which would make it more like air.
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