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Overwatch, when used properly can be tremendously effective
It's one of the few things a Marine player has at his disposal that gives him a distinct edge over the Genestealers. To be able to fire at a Genestealer each time it performs an action within your line of sight and fire arc can inflict huge casualties on the Genestealers. The only trouble with Overwatch as it stands now is the possibility of jamming. If you're like me, that happens a bit more than you'd like.
We've found Overwatch to be too powerful actually
That's right. You read that correctly. It took my friend and I a few games to realize that this mechanic is just a bit too powerful for our liking. Most of the shooting inside hulks is at close range for the most part and you don't often get a chance to use Overwatch to it's fullest effect. When you do though, it's utterly brutal for the Genestealer player.
That's both good and bad for both sides. Genestealers can often get close enough so that they only have to suffer a hail of storm bolter fire for a couple moves before being able to get into close combat. Marines, despite the cramped corridors, still have a chance to engage Genestealers at a distance where their odds of survival are the best.
We decided to change Overwatch to match our interpretation
Overwatch as it stands has a range of 12 squares. That's quite substantial when consider the restrictive confines of a hulk. Sure Marines have all kinds of targeting optics and such (when their machine spirit is working), but the environment inside a hulk can be temperamental at best. Add to that a Genestealer is not going to casually stroll down the middle of the hallway at a leisurely pace either. They're screaming down the hallways like a bullet giving Marines even less time to acquire their target.
To pick out a lightning fast target in those conditions would be nearly impossible on a good day... at any range really. Anything past arm's reach is going to be tough to sight in on and take down for even the best shooters who are firing reaction shots.
We have shortened the effective range for Overwatch to 6 squares. Anything past that and a Genestealer just moves in and out of a Marine's sights too fast to be able to effectively shoot at it. Once it has closed in though, a Marine is much more likely to be able to fire off an effect shot even when rushed as he would be.
This is not for the faint of heart
This limited range for Overwatch means the Genestealers are going to exploit this shortcoming. No more than they would the original range though either. Now the Marine is faced with some tough decisions when it comes to shooting. Does he close in and go on Overwatch knowing his effective range in Overwatch is short or does he stay further back in relative safety and use Command Points if available.
This was not the only way we looked at "solving" this, but we found it to be the most simple solution that did not introduce any new game mechanics or complex rules for varied ranges and all the other factors involved. Marines still shoot like normal, Genestealers still move like normal... you're just not going to be able to bring down a Genestealer at long range when it comes to going on Overwatch now.