RE: Best Slug for 20 ga Hastings ?
You need to find what your barrel/gun likes. My 20 ga. 870 slug gun patterns Federal 3" 5/8oz barnes expanders the best. The Hornady's are a close second with lightfields third and then Winchester platinums followed by Remington'. Funny how a Remington gun does not throw it's brand ammunition.
You need to spend the money and have a serious day at the range. The only way you will know.