Saying you can get 100 in a 10" every time, at what distance would you stop using 7.5 lead out of a .410?
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I agree with Spuriosity. See what range you can get the 100 in the 10, keep that as your max range, and go forth slaying turkeys.
I know without a doubt that 7.5 lead shot will kill a turkey @ 35+ yds. I've done it and seen it done several times.
(http://i567.photobucket.com/albums/ss115/earljeff/KansasTurkeyHuntapril09014.jpg)
This Kansas Rio fell to a load of 7.5 Remington express from my 16ga @ app 36yds.