So for fun, I wrote up a little Monte Carlo generator for determining the probability of destroying vehicles based on all the relevant parameters. While it is trivial to calculate whether a single shot will destroy a vehicle, if you want to do it properly you have to worry about the things like multiple weapon destroyed results leading to destruction and that all member of a unit fire at once. Instead of going through and trying to rigorously calculate the results, you just simulate the process a hundred thousand times. So the question I asked is how many times does the unit have to shoot at the target to get a destroyed result. The results are shown as a cumulative probability so you know that for example if you shoot at the vehicle 5 times it has some probability of being destroyed. It could have been the first shot or the last shot.
So the first thing we will look at is the difference between a standard 1 weapon razorback and las-plas razorback which 2 weapons adds to its damage capacity.
This plot pretty much shows the one of the problems with 40K right now. Here you have a elite warrior with line of sight and weapon in range with an anti tank weapon firing on an APC not a main battle tank. You do not reach 80% probability of destroying the Razorback until 10 missile shots. You can see that the extra weapon buys you 1 more missile shots on average before being destroyed. The mean number of shots to kill the razorback is 7.3 missile. This helps to illustrate the weakness of a unit like the tactical squad in this meta. You spend 170 points and get a heavy weapon that will not even on average kill a 50 point tank in an entire game. So in an army with 12 razorbacks you will need 87 missile shots to kill all those transports on average. Now 6 tactical squads are not going to bring that much firepower in 12 turns so you need devastators but things get a little worse since they fire missiles in groups of 4.
Now lets compare what a 4 Missile Launcher devastator unit does against razorbacks compared to the Rifleman Dreadnought.
With 4 missiles at a time you get destruction at 80% of the time after 3 sets of shots but this means 2 more missiles used than with the individual shots. The mean in 2.24 turns of shooting. Now if you have 12 razorbacks to kill it will require 27 turns of devastator shooting. This is 50% higher than your 3 devastator squads can deliver in the 6 turn games assuming that they take no damage (which is not going to happen since 12 HB psibacks kills 9 marines a turn). So their 600 points kills 160 points a turn and your 700 points (you are going to want some spare marines if you expect to have any missiles left after the first turn) kills 50 points a turn.
The Rifleman dread under performs the devastators where it takes him 4 turns to get the 80% point on the razorback. Add in Psibolts and he does better due to the twin linking.
What do you guys think? Are the light vehicles to tough. Should these tanks really survive 8 missiles shot by the emperors finest.