Warning: This post contains Mathhammer. But not much, and only at the end.
Non Twin-Linked Weapons
Let’s get non-Twin Linked weapons out the way first. You have a few markerlight hits on your target enemy unit – should you spend these to increase BS or to reduce cover saves? Let’s assume that your enemy will not get an armour save, as it keeps things simpler.
Well, start with spending a markerlight token on +BS. An increase in BS of one results in your chances of hitting increasing by 16.6%. If you don’t believe me, there’s a table below. With me so far?
What if you spend that same markerlight on reducing cover saves instead? It’s the same story as above. No matter what the cover save is, reducing it by one makes the chances of passing it fall by 16.6%.
In other words, for the most common situations, there is no difference between increasing BS and reducing cover saves with markerlights.
Since, on a real tabletop, you are likely to run into complications like armour saves, increasing BS first is usually the best bet. In addition there is the psychological impact of hitting on 2s and forcing your opponent to roll more dice 
Twin-Linked Weapons
Here is where it gets interesting – if you run Broadsides, Gun Drones, suicide Fusion squads, Deathrains or indeed any Crisis Suit with a twin-linked weapon.
RULE #1: Unless you are a Gun Drone, always reduce cover saves before you start increasing BS.
RULE #2: If you are a Gun Drone, spend one markerlight token to increase BS, and the rest reducing cover saves. If you have any left, go back to increasing BS.
RULE #3: If the enemy will get an armour save, don’t reduce the cover save below that!
Here is the mathhammer to help you work out what is going on. Once you start twin-linking weapons, changing BS has a non-linear effect on your chances to hit. At TL BS2 the improvement gained by jumping to TL BS3 is better than reducing a cover save by -1 (19% vs 16.6%), but after that the gains swap over, and reducing cover saves always has greater benefits than increasing BS:-
| Cover saves | ||
| Cover save | Chance of saving | Decrease in chance of saving from next best cover save |
| 2+ | 83% | 16.6% |
| 3+ | 66.6% | 16.6% |
| 4+ | 50% | 16.6% |
| 5+ | 33.3% | n/a |
| Non twin-linked weapons | |||
| BS | Hits on a | Chance of hitting | Increase in chance of hitting from next lowest BS |
| 2 | 5+ | 33.3% | n/a |
| 3 | 4+ | 50% | 16.6% |
| 4 | 3+ | 66.6% | 16.6% |
| 5 | 2+ | 83% | 16.6% |
| Twin-linked weapons | |||
| BS | Hits on a | Chance of hitting | Increase in chance of hitting from next lowest BS |
| 2 | 5+ with a reroll | 55.5% | n/a |
| 3 | 4+ with a reroll | 75% | 19% |
| 4 | 3+ with a reroll | 88% | 13% |
| 5 | 2+ with a reroll | 97% | 9% |
I’ve found that lowering cover saves is rarely useful, but that’s because most of my opponents are already rocking 3+ armor saves and I try to position myself to where no cover is provided from my Crisis Suits. It’s one of the reasons I dropped Markerlights from my lists; against MEQ armies, they’re just not productive enough to justify the cost, even with the ability to boost BS.
I am surprised at that, for two reasons. Firstly, every game I play with transports in it involves them moving up and popping smoke on Turn 1. That 4+ save effectively cuts my railgun shots in half and, as every Tau player knows, stopping them on Turn 1 can change the game. If a unit of 3 broadsides usually costs 283 pts (mine do), then I just lost 141.5 pts of shooting to that 4+ save. 3 ML hits (aka 6 Pathfinders, although 8 is better) costs me 72 pts and restores me to full shooting potential.
The second point is specifically in relation to marines. Your plasma rifles ignore their armour, so cover saves are vital to them. Take the cover away, and they might as well be Kroot under your guns! Don’t fire any AP4 shots at the same time as the plasma, though, to avoid wound allocation shenanigans robbing you of dead beakies.
For me it comes down to the Tau philosophy of concentrating fire on a single target until it’s dead, and markerlights make that much more likely to happen. I have a post about Pathfinders around here somewhere, where I describe how I use them.
I was under the impression that if you fired a units weapons you had to fire all weapons, not have the opportunity top pick and chose. One of the reasons I have been experimenting with targeting arrays lately.
That rule only applies in assault – “All engaged models fight with their full number of attacks”, but in shooting you can pick and choose which weapons you fire. Enormously useful when shooting at eg Termies with Fireknives. “Missile Pods off.”
Two thumbs-up for the post. It really clears the picture on what happens to your dice-statistics as you increase BS or decrease Cover Save. Thanx!
In other news, excellent blog. I’ve been following it for some time now and the articles are top notch. Waiting to see your new top-secret army!
PS. Cudos on Malifaux!!!