I'm a little torn myself on this because in any vehicle with less armor than a roadmaster (i.e. most vehicles) this is a pretty easy shot to make. But, in real life, hitting something in the engine block with any respectable round would likely be pretty devastating. It's gameplay vs. reality - in reality, it's totally legit. In game, it seems pretty OP.
Also, R&G doesn't say what "disabled" actually means. I'm fine with the shot "disabling" the vehicle, but I think that the exact effects of "disable" are where we get into the meat of the shot. I think that it should perhaps be akin to some of the similar effects for called shots against a metahuman - something akin to Buckled (No movement - effectively - for DV-Body Test hits Turns), for example - though such effects imply some self healing capability of the vehicle (maybe technobabble it as rerouting power or compensating for a disabled subsystem). But the most logical, and most devastating, result would be that the vehicle can't move until at least one box of damage is repaired, which would effectively end a chase or getaway (though being out of commission for even a single CT probably would too). Alternatively (and less usefully for this particular case) you could make it equivalent to the Vitals called shot and just add +2 to the DV. Maybe up the DV bonus to compensate for the vehicle's increased condition monitor, but this keeps the shot more inline with the called shot penalty.
In short, I'd totally hate to have this used against me, but as written in the book, I don't see why it wouldn't be used all the time. So I'm fine with any GM modification to the rule you choose to make. The effect I want is obviously to hurt the rigger if he happens to be jumped in, but also to either keep the van locked down for a bit so that Ikiryo can't use it to get away (Mercer is unaware that he has received a fatal dose of lead) or force someone out of the van to present a new target.
Barring significant change to the battlescape outside the church, Mercer's plan going forward is to move closer to the church, tag the Americar with a tracker, and then - I don't know. Either join the fight in the church (most likely) or try to crack open the Roadmaster to get to the soft fleshy bits inside. Depends on how things progress outside - the gangers might solve the Roadmaster problem for him (though I tend to think they are more likely to meet a gory demise as the rigger brings more of his toys to the fight). At 20-40 m/turn, he's got a lot of time to consider his options.
If you'd rather just not deal with this particular called shot (or opt to make it similar to the Vitals called shot as mentioned above), I can change up my actions to be
Free: eject clip
Simple: Load tracker clip
Simple: Shoot Roadmaster (It'd use the same number of dice; no -2 for called shot, but -1 for no called shot and -1 for no image mag)
As an aside, tracker rounds do damage equal to a regular round, so 15P -4 AP for Vera. It won't hurt the Roadmaster but will still stick. I just bring this up in case I need to shoot something more fleshy before I've had a chance to swap the clip.
