I have a Dryfire system and have been using it to practice doubles which I haven't shot for ten years due to going to a release trigger in my Alfermann. After converting it to release and shooting it for a while I reached the point with the almost no trigger movement to release my Alfermann, I would not move my finger enough to set the trigger on my Beretta for the second shot. I blamed the Beretta trigger for this and finally traded for a Kolar and had the same problem. And I bought the Dryfire unit to let me practice shooting doubles at home where I could just concentrate on the trigger set without the frustration of screwing up on the range and creating other problems.
The Dryfire requires you to enter all the vital info about your and your shells, such as POI, choke, and shell speed and pellet size It provides you with a lot of feedback about what your doing. The one drawback I've found is there's no recoil. The first time you go the range and shoot, you think your gun is going to jump right out of your hands for the first 5 - 10 shots. It takes a few times going to the range and this goes away.
Big advantage is you save the cost of a box of shells and a round of targets. Plus you don't have to bundle up to shoot. You can shoot in a tee shirt.
Troy Ellis