I realize most of the posts here are for rifle an pistol however I am mainly a shotgun guy who shoots skeet every week. I have a Browning XS Skeet O/U in both the 12 and 20 gauge. I primarily shoot the 12 gauge with a sweet 3/4 oz. load (a 28 gauge load) that I've been reloading for years.
By using this load for all my shooting in 12 through 28 gauge I save a lot of time and money at the reloading bench. I have Briley tubes for the gun in the 410 so I am set with the setup for all my shooting. I set up my three Mec 9000H reloaders years ago and have never had to touch it. I use Winchester AA hulls in all gauges and have thousands of the old compression style hulls in 12, 20 and 410. My daughter shoots my 20 gauge. I also have the 20 gauge reloader set up for a 3/4 oz. My friend Lenny shoots a 20 gauge and I reload for him also. He started last year and bought a couple cases of Winchester AA 20 gauge to start.
A problem arose as I was reloading the 20 gauge shells. My shell, that my daughter shoots reload with a perfect crimp however when I reload Lenny's shells I end up with an opening in the center of the crimp that allows shot to escape. After making a multitude of adjustment I was unable to get a crimp that I liked. Finally I worked on it and got a perfect crimp on the shells Lenny has. I then loaded my hulls and got an overly tight crimp that spiraled.
The light went off. Winchester has shortened the hull length on the newer hulls. Even though the load data has stayed the same I now need to make an adjustment to the final crimp stage of the reloader every time I switch from my old hulls to the new hulls. I found that backing off on the final crimp screw by one turn on the older hulls I can now get a proper close on both.
I guess if I had taken a caliper to the hulls I might have found this but with plastic hulls it's hard to select a shell that has opened up perfectly and is standing at full extension. Not sure if anyone else has run into this so if you have older compression formed AA hulls and the newer AA hulls and are having trouble getting a consistent crimp, this may be the issue.