Slab slugga is great for sheer damage but beyond the first shot your other bullets will likely go pretty wide of the mark. Zip slugga is pretty sweet but I find the overall damage too low for my taste, though it's ability to drive your opponent into cover quickly shouldn't be underestimated. Dakka slugga is probably the best way to go, with the Sparky Slugga being the overall best in my opinion. The Siz-Shoota, frankly, needs to be reworked heavily by the devs or just have its LP cost significantly reduced as it's the least useful of the pistols. Pokkit Rokkit and it's Unique version aren't bad for stunning an opponent but otherwise aren't that great (though watching all the Marines rolling away frantically from that slow-ass rokkit is pretty funny).
The correct answer is Dakka slugga or its unique Sparky slugger (best in game).......simply because per clip they can deal the most damage out of any sidearm with a pretty nice DPS. Zip slugga is fun and I have not done the math, but would suspect it is the highest DPS(damage per second). But, I do know it is not the highest total damage per clip.
I've actually found the zip slugga to be the most effective at taking down a Space Marine's shield, one clip and it's pretty much disabled.
A full dakka slugga will take out a shield, perhaps you are missing as it does have a bit of recoil/accuracy issues.
Probably the accuracy as I generally don't use a sidearm for much other than to soften up a target as I charge at them, or to discourage them from attacking while I get a new rokkit loaded.
So I did some basic math I'll share with you guys about the standard bullet pistols especially how the six shoota measures up to them: Six Shoota D - 30 Pen - 80 Reload 1.5/1 Ammo 6/36 Firerate 400RPM 6.6666RPS DPS per clip 199.998 Time to empty clip 0.9s Total damage per clip 180 Slab Slugga D - 70 Pen - 80 Reload 2/1.5 Ammo 7/28 Firerate 157.89RPM 2.6315RPS DPS per clip 184.204 Time to empty clip 2.7s Total damage per clip 490 Bullets fired/damage done in 0.9s - 2/140 Dakka Slugga D - 36 Pen - 80 Reload - 2/1.5 Ammo - 20/80 Firerate 363.64RPM 6.0606RPS DPS per clip 218.1816 Time to empty clip 3.3s Total damage per clip 720 Bullets fired/damage done in 0.9s - 5/180 Zip Slugga D - 16 Pen - 80 Reload 2/1.5 Ammo 40/160 Firerate 750RPM 12.5RPS DPS per clip 200 Time to empty clip 3.2s Total damage per clip 640 Bullets fired/damage done in 0.9s - 11/176 What I was trying to show was overall stats for each pistol, with the single line item of Stats vs A Six Shoota's active time. Obivously with reloads included the six shoota falls behind fairly quickly with the frequent reloads. But solid stats wise, the Six-Shoota is On Par, if not superior in most stats given the 1 second window. Of course this doesn't take into account missing, headshots, windup time(I think dakka and zip might have a slight delay if I recall) and a myriad of other outliers but it does give some baseline knowledge to work off of. Long story short, if I'm doing my math right and interpreting it correctly, the Six Shoota is not worse than any other pistols, given small windows to use it. This is why it is my opinion that the Six Shoota is the best point blank melee defense pistol specifically for Shootas, especially those using a Shoota, Big Shoota etc. Since you have a rifle, you don't need the pistol for ranged combat, but you can whip out the Six Shoota and weave in quick shots during melee where the windows to fire a weapon are tiny. The window the a DBash opens up is probably about exactly the amount of time you could empty a six shoota into an enemy.
As someone who tried using the Six-Shoota from early on (just cuz I like revolvers), I can say that in practice it's not as good as the stats make it seem. Emptying a clip is very easy and quick to do, unfortunately it's amazing how many times you'll miss due to opponents rolling out of the way. Unless you're quite accurate with twitch-shooting in close quarters, it can become quite useless very quickly. And once you have a Dakka Slugga unlocked, the Six-Shoota loses much of its relevance as the Dakka Slugga requires less aiming ability and can take a Marine down in about a clip. Not to mention the effect on players when there's a blitz of bullets coming their way, they often scatter so it has it's own version of Suppression in a way. Plus there is no windup whatsoever for the Dakka Slugga or the Zip Slugga, and once you get Mastery it costs the Dakka Slugga costs the same as the Six-Slugga. This is why, essentially, no Ork player will ever use the thing for longer than a simple test run. I've honestly never even seen anyone else using it past that.
Pffft...if I'm going to a quick and dirty "empty the clip" pistol, I'm putting on the One Shot Slugga. Feel like Dirty Harry with that thing.