Rocket HE kill values
 

I compared the HE payload of rockets in Peter Chamberlains book Mortars and rockets and Steven Zalogas book Red army handbook against your values
I used y=1.072^x to give a line of reasonable fit, and then brought HE Kill values back to this curve.
That showed several rocket HE Kill values are exceptional.

Notes.
15cm Rocket high HE Kill value may be because of schrapnel, Chamberlain says,

Just behind the venturi was a base canister holding the warhead. This configuration was chosen to ensure that the rocket motor and casing would be added to the warheads explosive effect-in conventional rockets the motor tended to fly off in one peice.

Soviet M-8 has a Fragmentation head, So the German copy 8cm Rocket also does?
USA M8 has frag head.
Flame Rockets high HE Kill reflects flame effect?

M28 is not an exact copy of 28cm so lower range.
4.6in Rocket range restricted by launchers elevation.
Panzerschreck rocket has HE payload of .667, bazooka rocket has .227, both have HE Kill of 4?

Re: Rocket HE kill values
 

I'll take a look at the data when I start OOB work again later in the year.

Don