As far as I can see from RaW the only way to get a wheeled vehicle to get out of control because of a hit to the tyre is to be either very lucky (and roll righteous fury) or to first reduce all the integrity to zero and then get critical damage.

An Example:

A sniper takes a Called Shot with his long las set to overload mode at a rear wheel of a Viator Desert Crawler (featured in Forgotten Gods). Because he takes the Called Shot he does not get any bonuses from the Accurate weapon trait (no aim action involved). He luckily hits the tyre. He has Mighty Shot and high BS totaling to a damage of 1d0+8 with Pen 3. He rolls a 9 resulting in 17 damage with Pen 3.

Now the Viator starts with AP 14 at the rear and 25 integrity and just gets 6 damage from this rather heavy hit to the tyre. That's it.

To me it somehow feels that a Called Shot to a tyre should have a better chance than that to cause serious problems to the driver. At the least the AP of the tyre should be much lower than the standard AP on the hit facing. And if getting any damage the driver should be forced to roll a Operate-test against getting out of control or something.

What are your thoughts on that?

What's stopping the sniper from Aiming before the Called Shot?

I agree that the AP should be reduced - and this is where the GM's prerogative comes into play, or you end up with a Shadowrun situation where there is a rule example for almost every single thing you could conceivably do, instead of a framework that allows the players/GM to arbitrate on what an action should involve.

Not trying to start any kind of system/edition war, both have their pros and cons, I'm simply saying, if something doesn't fit in with the game that you are running, that as the GM - providing it's not out of the blue, it's consistent and the player's as a whole are happy with it - you can add/remove or alter rules as you see fit.

What's stopping the sniper from Aiming before the Called Shot?

Good question! In my specific situation he just had no time to aim before the round he attempted the Called Shot (which is a Full Action).

I agree that the AP should be reduced - and this is where the GM's prerogative comes into play, or you end up with a Shadowrun situation where there is a rule example for almost every single thing you could conceivably do, instead of a framework that allows the players/GM to arbitrate on what an action should involve.

Not trying to start any kind of system/edition war, both have their pros and cons, I'm simply saying, if something doesn't fit in with the game that you are running, that as the GM - providing it's not out of the blue, it's consistent and the player's as a whole are happy with it - you can add/remove or alter rules as you see fit.

Thanks! Of course in the end I will adapt things to fit my story. I'm just sometimes a little afraid to provide such precedents to the players, out of two reasons: I don't want to break game balance and if I let them do it once then they will potentially use this specific action to try to stop any kind of vehicle in the future (which could become rather boring).