Any chance you can post some photos, because there is no way the 46 gr Round Nose BT fit in my SST. Also don’t have an Eagle Vision Mag but neither will fit the FX side shot magazines.
Hm, I just toke my Impact apart because tomorrow a friend of my can re-cut the peek valve poppet on the lathe. (That was my mistake, I designed the poppet head 9.1mm vs 8.1 mm. I would never know shooting pellets that the airpath is clogged, this issue came up just recently only when I started shooting slugs).
I have a original FX single shot loader, got it with my Impact Mar.2020. I assume all these shall be made to same dimensions....unless they changed the design in mean time.
Yes, the ball nose 46 gr slug is longer then the SST - inner slider total width, but the housing is slotted from the back (probe side) and the boat tail have enough clearance to slide in.
Now, you give me your secret

.......what you using a pin probe or a pellet probe?
I have no slug power kit, only a Huma pellet probe (I am trying to replace that one) and one home made pin probe somewhere from spring 2020, a very early concept built (and this is a reason I never used it before).
The 44gr boat tail fits the eaglevision mag but the spring need to be tightened in order to rotate 28 slugs nice and smooth.