I never had any real succes with the slower powders like Herco and Blue Dot in my slug loads. The velocity was always erratic and so was the groups. The faster powders like Unique and Red Dot gave much more constant, albeit slower velocity and grouped much better.
Spurred by BT's Fury slug test, I loaded up a couple of primed 3" shells with my 570 grains Brenneke clone on top of 40 grains Steel, a x12x seal, a 1/2" felt wad and finished with a roll crimp.
Fired over the chrono they gave an average velocity of 1420 fps with about 30 fps variation. This looked promising!
I then tried a slightly reduced load (38 grains Steel) in once-fired 2-3/4" shells. I didn't use a chrono but decided to try for a group right away. The first two shots felt alright, but the third had a telltale double report, indicating a hangfire/blooper. I decided to stop right there as I already have one shotgun barrel with a sack near the muzzle due to a blooper.
Thinking that maybe a bad crimp, combined with the thin skivred edge of the shells was responsible for the erratic velocity, I tried the same load in a shortened (2-1/2")shell with only the x12x seal under the slug and a nice, solid crimp. The first shot read 1200 fps the next 1350. Obviously something was not right here...
I began suspecting that the primers (CCI 209) was not up to the job. The 3" shells had worked flawlessly but they were from a batch of primed new shells someone once gave me and I have no Idea of the primer used, and since I haven't been able to find any 209 magnum primers over here, I decided to try a duplex load instead.
Now, duplex loads are something most'internet experts' warn against, but I had used them with good results in 577/450 Martini Henry loads back when I was young, immortal and didn't know any better So, throwing caution to the wind, I tried the same setup as before, but this time with only 25 grains of Steel over 5 grains of Red Dot for a combined powder weight of 30 grains. Lo and behold, this time the average velocity was 1290 fps with a spread of only 24 fps for 4 rounds!
It seems as if I have been wasting a lot of unburnt powder in the quest for speed. Next step will be to load a batch and see how well they shoot. All my best groups have been made with fast burning powder and have all been subsonic when leaving the muzzle. It shall be interesting to see if the slug still will produce decent groups with the speed in the 1300-1400 fps range. I'll probably end up with a sore shoulder, but science often demands sacrifices