I know this is on calling but I like to add that, you need to also test your pattern on which ever Ga shot gun your going to use because if your a great caller and get a nice Tom into range and can't him him all the calling in the world isn't doing any good .
Each shell shot is something you chose on how well the pattern is out of your type of gun being used . you have a lot to chose from in Turkey loads from NO# 4's , 6's and the differnce there is amount of BB's in each shell 4's are less projectiles and 6's are smaller and more projectiles .
Size of shell
2 3/4
3"
3 1/2
Any of the above is going to drop your bird as long as you know your pattern of each shell .
Question asked is why do I need to pick from those
Again you have a choice and each one of these have more shot in them as well as each shell provides you with more range and comes in many choices of shot .
Gages
16Ga.
20Ga.
12Ga.
again this is a choice to make and each comes with bigger patterns depending on size of choke or modify from manufacturer .
Chokes
Not every gun comes with interchangeable chokes this choice is used for the distance you are wanting to shoot as well as pattern .
Fixed chokes are made when gun barrel is formed anything from MODIFY, IMPROVED, FULL , and this goes along with chokes that are interchangeable in newer guns .
Not to make you think wow thats a lot to chose from my saying is pick on and practice it is good to be sure on how your choice patterns no matter the Gage or shell choice so best of luck hunting the Elusive Tom turkey