The recent FAQ removed the ability of Deathwatch units to use Special Issue Ammunition and the new Bolter Discipline rule in combination. This has led many Deathwatch players to ask which is better to use in which situation? I've run the numbers and here is a rough guide to what Bolter type to use against a variety of foes.
In each of the tables below, the average number of unsaved wounds caused is listed for each ammunition type against a range of common model types. Each example assumes that the shots are being made out of rapid fire range, so single shot for SIA and two shots for Bolter Discipline. In addition, the effects of re-rolls (Watch Master, Watch Captain or Mission Tactics) are not taken into account. It also assumes the unit did not move, in order to gain the benefit of Bolter Discipline.
In the section for Hellfire bolts, the first number is for vehicles, while the second number is for monsters.
EDIT- This information only applies at long range and not at rapid fire range, where Bolter Discipline has no effect. I did not feel the need to spell this out, but a lot of people have been commenting to point this out in other groups that SIA is better at rapid fire range. I didn't feel this was necessary to specifically point out as I thought it was an obvious conclusion, but do so now just in case there was any confusion.
EDIT- This information only applies at long range and not at rapid fire range, where Bolter Discipline has no effect. I did not feel the need to spell this out, but a lot of people have been commenting to point this out in other groups that SIA is better at rapid fire range. I didn't feel this was necessary to specifically point out as I thought it was an obvious conclusion, but do so now just in case there was any confusion.
Deathwatch Veterans
The examples below assumes 5 Deathwatch Veterans armed with Bolters.
As can be seen from the table, the Bolter Discipline is better or equal to Vengeance Bolts in almost all situations. Only for models that are T5 or above (and not vehicles) are the Hellfire shells a better option. Once you factor in re-rolls, the Bolter Discipline is superior, simply due to a higher number of shots, so more chance of getting to use re-rolls.
Intercessors
The examples below assumes 5 Deathwatch Intercessors armed with Bolt Rifles.
Again, at long range, Bolter Discipline is superior than all other ammo types, except where dealing with T5 or above models.
Deathwatch Bikers
The examples below assumes 5 Deathwatch Bikers armed with Twin Bolters.
This table is similar to Veterans, but has double the number of shots. This table will also apply to Terminators and Veterans armed with Storm Bolters.
Conclusions
The numbers show that at long range, it is always better to use Bolter Discipline for additional shots against the range of targets listed in the tables above.
When you add in the benefits of re-rolls to hit, either from a Watch Master or Watch Captain, the benefits of the Bolter Discipline are greater, simply due to the higher volume of shots, giving you more opportunity to benefit from the re-rolls.
The only time to not use Bolter Discipline is against models that are T5 or higher. Against such units, the Hellfire Bolts are generally superior, as wounding on a 2+ beats the additional firepower of the Bolter Discipline.