Any of the 3 or 3.5 shotguns are capable of killing a turkey at 40+ yards as long as you find a shell/choke combo that the gun shoots well. To just pick a choke off the shelf it can be a crap shoot. I have tried numerous shell/choke combos in my turkey guns before finding one that I like and that gives me the pattern that I feel I need to ethically take a turkey at 40+ yards. Two combos that have worked for me time and time again on birds out to 47 yards are:
Browning Gold 3.5 w/26"barrel/MAD MAX .670 choke/Winchester 3.5 Extended Range #5.
Remington 11-87 Super Mag w/23"barrel/Kicks Gobblin Thunder .655 choke/Winchester 3.5 Extended Range #6.
My advise is to pick a shotgun that feels good to you and try several shell/choke combos until you get one that performs to your standards. If you don't mind paying the money the Rhino choke and Nitro custom ammo is another avenue. They have been really producing some awesome patterns and Nitro Company has tested alot of load and choke combos in all of the major shotguns.
Here is a link to Nitro's web site.
http://www.nitrocompany.com/pattern.htm#recommended%20shells
Good luck! I will be trying out the Rhino/Nitro combo before this spring season.