Matt, your post has been here a while with no responses, so here goes what I know. I am limited to Parkers, can't answer for others, and this is a PArker forum.
Choke lengths: in Parkers 2" to 4". 2 1/2 to 3 is common, and varies by amount of constriction.
Convergence: Its not like a double rifle set up to shoot to the same 1" at some distance. Shotguns are set up to deliver most of their shot in a 30" circle at some yards distance. 40 yards is standard patterning distance but some meant for close shooting were patterned at 25 or 30 yards. The less constriction the bigger the spread but what Parker called the killing circle is defined as a 30 " circle , or effective "pattern". You can attempt to determine the distance of convergence by sighting through the centerpoint of each bore. Ideally you should be able to sight down the rib and the shot pattern from both barrels should be centered in the same circle. However, some trap guns, including Parker trap guns, shoot higher than the aiming point; a built in lead for a rising target.
Its an interesting question, I've never been inclined to sight through mine and find out. Probably wouldn't make a bit of difference.....I can't hit anything anyway.
Maybe some shotgun ballistics experts will chime in now that I've got it started.
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