In message , Eunometic
writes
I doubt the Russians would produce a completely impracticable weapon.

However, not all their concepts have proved effective when tested, and
many have performed poorly when exported in a reduced-specification
version

Almost certainly a modest degree of directional control (perhaps turn
rates of 1 degree per second) is possible

How? It's in a supercavitating bubble. Depth keeping and an
approximately straight line is the best it will manage without dumping
the bubble.

if only to keep the missile
on course, homing guidence at full speed might be difficult due to
the gas cavity

Try "impossible". That supercavitation effect blinds any possible
sensor, plus the location for the sensor array is taken up by the gas
bleed.

Similar set of problems to polyox injection, except supercavitation at
least gives a lot more speed.

and rocket motor interfering with both passive and
active sonar but that wouldn't prevent the missile being equiped with
an inertial guidence system able to take the missile to within close
range of the target where it either slows down for a 'look' using
conventional passive or active sonar or it detonates a large (possibly
nuclear) warhead.

Shkval was designed as a reactive weapon to throw a packet of instant
sunshine in the general direction of an enemy who had revealed himself
by firing. It's now being marketed as a conventional weapon intended
to... well.. go really fast.

Even a cheap inertial guidence system would have
drift rates of at most 20 meters per minute; given its speed of well
over 300km/h or 5km/minute so an attack on targets 25 km away would
place the missile within 100 meters of the 'enemy carrier' or 'sub'.

Plus five minutes of movement by the carrier or submarine, who has heard
this weapon coming. (Shkval's range is typically cited as only around
6,000 yards, for reference)

Even attacks using WW2 shoot and forget collision course type aiming
with spreads of torpedos would have a high degree of success given the
enormous speed of the missile preventing evasive manouvers.

In all this, you assume perfect targeting by the submarine, of course.
How precisely can you judge bearing, range, course and speed of a
submarine from 25 kilometres out?

--
He thinks too much: such men are dangerous.
Julius Caesar I:2

Paul J. Adam MainBoxatjrwlynch[dot]demon{dot}co(.)uk