I love shooting the Speer plastic bullets w/plastic cases in my Smith revolvers. I set humanoid targets up in the garage and run by shooting them as quickly as I can. Its excellent "shoot on the move" training and a lot of fun. I'm just surprised the accuracy is not better, as I might get 6" groups at 20 feet. I would think the accuracy should not be much worse than standard ammo, so is there some thing I'm doing wrong in not consistently apply the same pressure in seating the bullet, or some other trick to get decent accuracy?

whats the bullet dia ? my guess is that it is slightly undersize. Look and see if rifleing marks are on the bullet.

They make training easy and fun. 6" isn't "bad" grouping for a human target.

The accuracy you're getting is about right for those. They won't drive tacks, but they're still a lot of fun and good indoor practice.