Nielsen Slugs

Shooting the Nielsen 54g slugs out of a Daystate Safari .30. I can’t get them to group at 100 yards no matter what. Best group is about 3-4 inches. With the 44g JSB the gun I’ll do quarter size groups all day long. If I use them over 200 yards they group better than pellets do. I had read someone else noticed the slugs doing better the longer distance you shot. Groups at 50 yards were horrible. Anyone else see this shooting slugs?
 
The 49.5g goes into the barrel really easy. Almost felt loose. I couldn’t get them to group smaller than about 5” at 100 yards. Didn’t bother shooting them past 100. The 54g go in pretty tight. I only shoot target paper past 100 yards once or twice a year. I have up to 100 yards then 205 yards is the next step up. Past 100 I’m shooting across a lake. It’s a half mile swamp walk to get around the lake. I have 2 steel plates across the lake. I usually shoot paper across in the spring and fall, easier walk, or I bribe my daughters to go hang them. My best group is 5” at 230 yards with my FX Boss. I haven’t shot the Safari at paper yet at that distance, but judging by the hits, the group looks at least as good as the Boss.