SECRET WEAPONS
As an action-adventure and SF novelist, I can hardly resist speculating about futuristic weapons.
V3 technology
Hitler's V3 long range artillery used side charges set off in angled tubes to periodically boost the velocity of the projectile. However, part of the extra force will be increasingly spent backwards rather than encouraging the projectile.
The design has other shortcomings. Access to the side charge ports requires considerable tunneling, whereas if there were no side charges a simple drilling rig could prepare the ground for the tubes.
Recoilless V3
Instead of using a high-velocity nitro-propellant, in the breech of a conventional gun, use instead a lower combustion speed formula and make it part of the projectile, graded by combustion speed over its burning length. Thus as the weapon fires, the combustion speed can be slower than the final boost required as it nears the aperture.
Adding high-speed automatic doors to close off and vent 'spent' tube sections could further improve efficiency.
Since the acceleration is near-constant over the distance, pressures should be considerably less than the 'stage explosion' method. Velocity could be further improved by coating the projectile with PTFE. The final section only should be rifled.
Anti-aircraft
As an anti-aircraft weapon the V3 could work quite well, since the projectile is capable of reaching a considerable altitude. In this case an anti-aircraft projectile would be fired when detection systems calculated the optimum moment for interception.
Anti-stealth
One possible use is in detecting stealth aircraft. The projectile bursts at high altitude and disseminates an aerosol of a compound that produces an immediately recognisable signature when burned. The compound drifts slowly down. When incoming stealth aircraft pass through the thin aerosol, they produce a characteristic ion plume that gives an immediate indication of height and direction. (Ions reflect radar very well; once you spot an ion plume you can target your R&CC AA weapons at the start of the plume. Obviously, some software changes will be necessary.)
Compared with Railgun
The railgun has the advantage of zero emissions, making it harder to detect when it fires. However the recoilless V3 has the advantage of being self-contained, providing the ammunition doesn't run out. The railgun requires a small power station to provide its motive source, not an easy thing to defend unless it were deep underground.
Disadvantages of both
The relative immovability of the tube or rail. Range along the axis can be
achieved by varying the propellant quantity. Slewing the tube is not a good idea
because of the extremely high velocity of the projectile, and also because any
such method makes the device more vulnerable to enemy action. The Hitler V3
presented merely a 15 cm aperture in the rocky ground.
Therefore the
projectile itself needs to be steerable; hardly a problem in the age of the
'smart' bomb. (See later, 'steerable, intelligent counter-countermeasure
projectiles' ref. Doudy, McGuire, Houhn and Pushkin.) An onboard computer will
steer the projectile to the desired GPS coordinate. (Acquiring auto-GPS
clusters, Gen. R__)
Emplacement
Diamond-drill into granite. Use the rock itself to reinforce a relatively low-tech tube. Emplace multiple tubes so that only a nuke could put them out of action. (Even so, the automatic closure and vent doors will probably limit effects to the surface and near-surface)
Protection
The outer perimeter will of course be cruise/stealth-protected by the Kevlar(tm) helium barrage as noted by Goodyear Defence Blimp, Inc. (1998, further details unavailable)