Oakhonor,
Yes, you are correct. I think the possible difference is in the diameter. I had these dies made for a Rapid Air Weapons TM 1000 barrel, and Steyr LG 110 FT barrel, which are very similar to each other.
I'm not sure what the diameter of the Neilson slugs are, but I'm going to guess that mine are a touch smaller in diameter (.1775), which is probably why they shoot halfway decent through a choked barrel. Larger diameter slugs show the characteristics that you described with your guns.
Being that I'm stuck shooting Indoors at the moment as well, when I get a chance to mess with the 1720T, I'll be doing some indoor velocity strings and tests, and I'll do some preliminary group testing at 10 meters.
Stay safe, stay tuned
Tom Holland
Field Target Tech