Media:original-12-123441-3449790-snake-20feat-20speed-20vs-20ocelot.png
http://i.imgur.com/BOq6go9.jpg
Snake Height - 182 cm or 118 px
Panel Height - 192 px
2atan(tan(70/2)*(118/192)) = 32.47 degrees
http://www.1728.org/angsize.htm
Distance to Snake - 312.51 cm
I don't know what gun is being used, but the link says Ocelot is the one shooting. I'll use Ocelot's standard gun, the Colt Single Action Army Revolver, and the fastest sniper rifle in MGS2 (since I think the scene takes place during this), the PSG-1 to be safe.
Colt Army Revolver Velocity - 253 m/s
PSG-1 Velocity - 868 m/s
Snake Height - 182 cm or 97.05 px
Jumping Distance - 105 px
105/97.05 = 1.082x
182*1.082 = 196.92 cm
312.51 cm/868 m/s = 3.6 milliseconds
312.51 cm/253 m/s = 12.352 millseconds
196.62 cm/3.6 milliseconds = 546 m/s
196.62 cm/12.352 milliseconds = 159.2 m/s
Results
Snake Backflips Over a Bullet (Low) - 159.2 m/s, Subsonic
Snake Backflips Over a Bullet (High) - Mach 1.6, Supersonic