Single-shot adapter: http://www.loweyproducts.com/products.html

As for what size groups to expect at 100 yards, that's impossible to say. Shoot it at 25 or 50 yards to find the ammo it likes best and it will consistently shoot into MOA or better groups. By consistently I mean at least 8 out of 10 5-shot groups. Once you know what it will do at 25 or 50 yards you'll know what it SHOULD do at 100 yards if you account for the conditions properly and your skills are up to the task.