I managed to shoot them today, I didn't to take pictures of the results today but those will come. I tried 3 different velocity setting with the gk1.
750fps, 850fps and 900fps, all shot at 60yds as I turned up the power, the groups kept getting smaller.
At 750fps the groups were averaging 2"-3" inches. I was shooting off the golf cart sterring wheel with some right to left wind, 6-7 mph on occasion I got some that were touching.
At 850fps the groups were 1"-1.8.", the shots were more consistent, I noticed that I had more of them actually hitting relatively the same spot. But a few fliers, which could've easily been me.
At 900fps they shrunk considerably, I mean touching each other, or hitting the same spot. Easily the groups were an inch and under.
These slugs are very promising and I'm looking to do more testing, I shot the gun at 290b, tethered to my tank at the 850fps and 900fps groups. Putting out 59.6fpe at 900fps you would get about 6 shots before dropping to 844fps which would cause your slug to drop about 2 inches at 60yds. At 850fps at 290b you're gonna get about 8 shots before you drop to 800fps.
Like I said in the beginning I shot this off the steering wheel of my golf cart, at 60yds, while the gun was tethered next to me. And surprisingly these shot better than the 25.39gr hades, which have me 7-10" groups at 25yrds from 800fps to 920fps.