I've posted some of this in other threads but MY Uragan .25 barrel has NOT played well with slugs even with 4 diff slug weights/styles both stock and each being sized to 3 to 4 step downs. That said... I have not adjusted the reg or even the HST to try different speeds to accommodate diff slugs. I was just hoping things would fall into place. I have seen posts on the .22 doing well, and even the ,177.
I don't have a need for slugs, have just been curios. So while I'm sticking with pellets, my point of comparison would be that I'm shooting 34g JSB at 890fps (very nicely!). All the slugs I've tried are at least a little slower with larger groups (some way larger) on average at 75y and 100y, at the settings.
My gut still says it's up to the barrel to decide, I just can't give you a positive ref on my gun vs slugs