Marksmanship runs are my favorite way to play DoomRL, so this topic is very interesting to me.

Minimum damage dealable is 1, so you probably are better off this way anyway.

Disclaimer: These numbers are all from the wiki, so they may or may not be correct. My guess is that they are at least mostly correct.

Average base pistol damage is 5 (2d4). Cyberdemon has an armor rating of 4. So if you have no damage mods or SoG ranks then half of the time your damage will be reduced to 1 and you will do an average damage of 1.625 per shot.

With 1 level of SoB, your average goes up to 2.25. 2 levels makes it 3.0625, and 3 levels makes it 4.

So each level of SoB makes a significant difference. Adding Damage Mods has an identical effect. And against enemies with less armor they would have an even greater effect.

Out of sheer curiosity, how fast can you shoot an Advanced Pistol (S4)?

Base fire time for a pistol is 1 sec.

SoG: firing time reduced 20%

Fin: firing time is reduced by 10%

Speed mod: firing time is reduced by 10%

Base * SoG 3

1 sec * (.8 * .8 * .8) = .512 sec

Base * SoG 3 * Fin 3

1 sec * (.8 * .8 * .8) * (.9 * .9 * .9) = .373 sec

Base * SoG 3 * Fin 3 * Speed Mod 3

1 sec * (.8 * .8 * .8) * (.9 * .9 * .9) * (.9 * .9 * .9) = .272 sec

If we use these numbers and combine them with the above, we can get some damage/sec information.

If we have a soldier with a pistol, SoG 3, and the option of adding either 3 damage mods or 3 speed mods to his pistol, what is his best option?

Base:

4 dmg / .512 sec = 7.8 dmg/sec (8 dmg / .512 sec = 15.6 dmg/sec against unarmored)

Damage option:

7 dmg / .512 sec = 13.672 dmg/sec (11 dmg / .512 sec = 21.48 dmg/sec against unarmored)

Speed option:

4 dmg / .373 sec = 10.7 dmg/sec (8 dmg / .373 sec = 21.45 dmg/sec against unarmored)

It looks like on average you are always better off using damage mods. Damage mods give you a significant advantage against armored opponents. As the enemy armor decreases, the advantage diminishes.

It would be a little trickier to determine whether speed mods or reload mods would be better for DPS, but my instinct is that the reload mod would be better. Since your speed is already up due to SoG, there is a point of diminishing returns with more speed mods. However, if you have not taken any Reloader traits then you get the full benefit from a reload mod. I would probably use a magazine mod over a reload, though.

My assumption is similar with accuracy mods. Since pistols start out so accurate already I don't know if they are worth using more than once.

Of course, this is assuming that you have your pick of which mods to apply. More often you just have to use what you get. But looking at the numbers, I think I will still take almost any mod over a speed mod when given the chance.