Take this from a southern MI hunter who uses a smoothbore but has been a 'range master' for about 15 yrs during the "sight-in season'. I've seen several hundred different hunters (at least) and most over multiple years. You might say I don't have a horse in the race, but I've rubbed shoulders with more owners than the average joe.
Look at Lightfields. They have been the most popular specialty slug around here over the last 15 + years. The number of hulls and boxes in the trashcan don't lie. However, they are loosing ground in the newer faster twist barrels and to the newer very high (for slug) velocity loads like the Hornady SST/Win Supreme Elite/Rem Copper Solids etc.
You're Hastings Paradox barrel has a slower twist than what has become standard in slug guns. It's 1/35 instead of the more common 1/28 to 1/24 on most models now. It was also one of the FIRST rifled slug barrels available. You can assume then that it was designed for slugs that were common at and before the age of the modern sabot began. Since it came from France, I think the makers would have designed it for what they were used to and was common there. When I was in Europe in the 80s, French slugs were generally a Brenneke type design, or a brenneke like projectile with a hollow base similar to our Fosters (but not as large of a hollow) and NOT attached to the wad column.
With your twist anything might be useable, but after the Lightfields I'd start with the standard Fosters. I'd try Winchester 1 oz first, then Remington. The Lightfields always impressed me as a "foster in a standard plastic shotcup style wad". (I can't see why they command the price they do, considering this; but they do shoot). You will get leading with the standard fosters, but should have a good 10 rounds or so of accuracy life. The Winchester/BRI Sabot Slug (1 oz and the hourglass shaped projectile) is about the original Sabot slug. Gun Digest (1966?) had a large article on the original BRI's (Win didn't buy them out for another 20 years about). They were shot in smoothbores because that's about all that existed then. They gave results as shown by that article in the 3 to 6 in 100 yard area; they used plain and ventilated rib barrels mostly, some with sights. A great improvement over pumpkin balls and came in a lead projectile version for hunters and a zinc version for LE. The zinc slug LOOKED to have the same dimensions, so lighter weight, but higher velocity and better penetration on barriers and autos that cops would need. About 1990 (+ or -) when Winchester started marketing them they were pushed for the new rifled barrels and rifled choke tubes only, not smooth bore. I'd break slug history into 3 generations. Gen 1 would be fosters and brennekes, Gen 2 would be the Win/BRI Sabot, and Federal/Barnes "Expander' and Remington Copper Solids. Gen 3 would be the 'pistol bullet in a sabot' Winchester Elites and Hornady's. These were all out in there original offerings by the end of the 1990s. This decade has mainly seen improvements/modifications of these.
I hope you get inputs from Hastings Paradox users. Until then, perhaps this will give you a way to evaluate the various slug loads for a starting point.
