Quick question has anyone got a .25 crown and shooting slugs good

I have tryed my reg at 150,160,170then 175 and adjusted hammer on each and the max speed i can get is 880fps with 34gr i have a ernest rowe hammer weight kit and extra high huma reg ive shot this at 50yrds and it groups are about size of a fist , ive been told from neilsens they need to go well over 900 fps but how i know loads of people struggling with same issue thanks ste

You need a hammer weight to go past 160bar normally, otherwise there's not enough for e to open the valve and your velocity will drop. I covered this in great detail in a post recently, just search my profile for the thread.