After using a 12 ga. on turkeys for the last 52 years I'm seriously considering an Ithaca 20 ga. Turkey Slayer. Anyone have any experience or knowledge of effective patterns at reasonable ranges with any of the newer loads and shot (like TSS #7s) ? Opinions, please! Don't suggest any other guns, I'm a diehard Ithaca user.
I've got a 12 gauge Turkey Slayer and love it. Sorry I can't really chime in on the 20 gauge.
With the advancement of ammo and choke configurations, I'm certain you can find a good combo for the Ithaca (any 20 gauge). The right combo makes "reasonable range" same as you have now with the 12 gauge.
Carlson's offer a Tru-choke in .575. Or you can check with William at Sumtoy.
What brand are you looking at? If you are shooting TSS you may as well go to the smallest shot legal in your state. If using Apex or something loaded similar then a Carlson 575 will do you fine. If you are shooting Federal TSS then I would research the constriction a little more as the wad could cause it to be more picky. Remember you usually do not have to spend $65+ on a choke for TSS. Great choice in firearm as well!
I would echo the advice to shoot the smallest shot that is legal for your state to maximize pattern density and effective range, although I would not go smaller than 9s for a 20 ga. TSS 9s are far superior to 7s for turkeys as long as they are legal in your state.