I have tried the NSA and ZAN Slugs in .25 and neither was very accurate at all. 25.39 JSB's are extremely accurate so I tried slugs as close to that weight as I could get, but both were a no-go. One thing I notices is that both brand slugs fit loosely in the magazine, even when seated, where the pellets will seat nice and snug in the magazine.
 
Are they slug shooters? I'm interested in user input in all calibers.

I own the SK19 in .30 cal and it has done quite well with both, NSA (49.5 Grain) / .300 "Hollow point" Slugs and "Infinitewares2015" .30 cal (60 Grain) / .301 "Boat tail" Slugs (a vendor on eBay who produces his own slugs @ a great price and does a Buy 3 get 1 free deal with only $3.00 shipping!). They both have a snug fit in the magazine and produce high accuracy.

Granted, I'm only shooting within 25 yds. But, I pretty much get a lot of hole in hole accuracy with both. Honestly... this has been a cheaper solution the last 9 months than JSB's or any other mainstream brand of pellets. I get 200 slugs per unit with NSA & 100 per unit with the buy 3 get 1 deal on eBay, which saves a lot and allows me to plink quite a bit more. ;-)