My Slug Shot build is:
Martial Gun has a +7 racc augment on it from FoV
Anwig Salade has +10 racc, +5 ratt, +4 str, +15 ws acc
Mirke Wardecors has +10 racc, +10 ratt
Amemet Mantle +1 has +1 agi augment from FoV
Desultor Tassets has +7 racc, +7 ratt
Those are some impressive numbers Zulos, I agree with Wooooodum that your sets are very similar. It must come down to buffs and/or merits. Zulos has +10 more str in gear (flame ring and mirke body). But Wooooodum should have 3 more base str for a net of 7 base str in favor of Zulos.
Assuming there is +10 ratt on Zulos mirke (giving him the best choice for maximizing slug damage) Wooooodum should be down 1 ratt on body. The 7 str difference is at most 4 ratt, combined with the 1 ratt from body is 5 ratt. However, Jalzahn's ring more than compensates for that.
For weapon damage the 7 str will be at most 4 base damage during slug, but Wooooodum has 5 more AGI (1 base damage at the least during slug) and a weapon with 2 higher base damage.
When you add it all up, Zulos has at most 1 more base slug damage than Wooooodum and 1 less ratt. You two would have virtually identical ranged attacks (with 1 point of eachother). There is no way under identical circumstances that Zulos could be doing more than 1*5*3 = 15 damage more than Wooooodum and thats only if ratt is capped. The extra damage has to be attributed to better buffs, better merits (they wouldn't make that much of a difference) or some sort of defense down effect.
I still don't get the logic of coffinmaker, unless you're concerned with drawing hate from slugging too often. If your ranged accuracy is an issue at all martial is always going to be substantially better during slug shot. The fact that you can weaponskill at least 1 shot sooner with martial and still have higher acc than coffinmaker will more than outdo the 9 acc the rest of the time. Unless you have 95% racc at 100 tp with slug (I would be very surprised if thats the case). Obviously it is working for you, but I am curious what TP % you slug at. I have both coffinmaker and martial and I've never been happy with coffinmaker accuracy anywhere near 100 TP. Of course its less of an issue for me since coffinmaker only has 2 more racc than my martial.