I Found this from Jeff Phillps web site, hope it helps clear up some of the brining questions about store bought turkeys ??.......
*****Can I Brine a Store-bough Turkey?
Some folks will tell you to not brine a store bought turkey but, for the life of me, I am not sure why not. I have been brining store purchased turkeys for many years and some of them with as much as 12% solution added and it is NEVER too salty. I am not a big “salt” guy so I would not like it or recommend it if it wasn’t good.
The process they do at the factory does not result in a salty turkey.. not even faintly so. The brining you do at home does a much better job and if you follow my instructions of using 1 cup of kosher salt to 1 gallon of water for an overnight (10-12 hour) brine, it will be a more juicy, moist and tasty bird than it can ever be otherwise.
My recommendation is that you try to find a fresh, no solution added turkey if possible. If you can’t find that, then shoot for a turkey that has 8% or less solution added.
Once you do it one time, you will most likely never eat an non-brined turkey again.