The 2~3/4"#4/or/#4x6 Shot Has Dispatch Turkeys for 60+/ Yr,In 12Ga,16Ga,20Ga with a Full Choke, If your Shotgun has a 3"or 3~1/2" Chamber, I would use a Light #4/Shot load and a Full choke, No reason to get beat-up by recoil. Perfect your Calling to bring then in!

I used 3 inch #6 shot, copper plated buffered magnums. it was 1 and 1 1/4 ounce of shot. very loud and very hard kicking. killed the bird though! to sum it up, you dont need a magnum load. a friend of mine killed a jake with #7 1/2 shot out of his 12 gauge, and last year I shot a jake with 2 3/4 inch #6, and these were just plain old bird shot, nothing special.

Personally? The 3" and 3 1/2" ammo is way over rated.
I shoot 2 3/4" Winchester, hi-brass or hi-velocity #7 1/2 shot for turkey. I've never lost a shotgunned bird nor had to shoot one twice. One round does the trick.
In my opinion, those "magnum" rounds make people take unnecessary chances. I've taken several birds with #4 shot imbedded in them.
Get a bird within 35 yards, shoot a tight choke, aim for that spot on the neck where the feathers stop with a dense pattern. Just one pellet in the spine and the bird is yours.

If you have the time and a little bit of money, the best thing to do is try a variety of rounds at the range to see which one will pattern the best out of your gun. Different guns and chokes will all handle loads differently to a certain degree. Though I myself use 3 ½’s, you probably want to avoid using that stuff unless your gun won’t take any other loads well. You will get some more effective range out of it, but you pay a steep price in money and pain for it. Personally I would start out 3 in. No 6. It’s a good medium between the light stuff and the cannons. No. 5 shot is also potent turkey medicine. I’ve never been a huge fan of No. 4 shot. The bigger pellets may give you more penetration, but with turkeys penetration is not as critical as getting a good pattern. Put a couple of pellets in the bird’s spine and trust me, he’s not going anywhere. 2 ¾ will do the job just fine within 35 yards, as will 20 gauge for that matter. But if you have the 12, you might as well get that little extra bit, unless of course it’s too much recoil. The last thing you want is to become recoil shy from a Turkey gun. At the end of the day, the most important factor is whatever you feel comfortable with. Comfort means confidence, and that’s 90% of the game. One more thing, be sure to check your local laws on allowable shot sizes. I see many here recommending No. 7, which is nice load, but unfortunately illegal in some areas.

@Knife Freak, you should NOT have a Problem with you shotgun with MOD or Full Choke just patten some loads at 40Yd on 24"x36" Paper,with a 12" circle in the center and a Shoot & See Turkey Head target, then count all the shot in the Kill Zone.

Not taking thing one away from our colleagues here, but why don't you ask your shotgun? We can tell you what we prefer or suggest, but until you pattern different shot sizes and try different shell sizes you won't know for certain. Try shot sizes 4,5, and 6 in whatever different shell lengths your gun accepts at ranges out to 40 or maybe 45 yards and see what patterns the best. Picking shotgun ammo for you would be like picking a lady for you, chancy at best.