Thank you all for the kind words. We've had another couple of days of rain that have prevented the roofer from his work on the house and shop but it will happen.
I've confirmed with Federal that they are continuing to produce the Heavyweight shotshells. There was a rumor going around about their stopping production. I've always found the best way to get an answer is to go to the source, so that's what I did.
loads of the Federal Heavyweight #6 shot 20 gauge shells that I shot did well through the Swarm & Pure Gold .555 chokes and through an Undertaker .572 choke. They also did well through a Pure Gold .570. There was a difference in the development wads and the final production wads and that apparently made a difference in performance. I've talked to others who experienced the same bridging or crowding effect that you describe, CB.
The #7 shot load is an entirely different animal, at least in my somewhat limited tests thus far. I had a very surprising pattern with the SSX and that shell http://allaboutshooting.com/article_info.php?articles_id=300
I believe that a choke is able to shape that shot charge more easily due to the size of the shot and then stabilize it within the wad in the parallel section. There's also an opportunity to reduce any harmonic problems and allow the shot to settle.
One "secret" to shooting the Flitecontrol Wad shells is to have a very clean barrel and choke. Plastic build up especially can cause some serious problems with that very long wad. I have run some additional limited testing on some prototype chokes and some enhancements to existing chokes that hold some promise.
I'll post more results as soon as I'm back on the range.