BCN gate transit time question
 

Anyone with BCN airport experience care to help.

JFK to BCN via AirFrance landing at 10:10am. Can I catch a Ryanair flight 11:45am (so need to be at gate by 11:15???) to Prague? Next flight would be at 2:35pm. Cost about the same. Carry on bag only, US Passport

No chance, they use different terminals on opposite sides of the airport

You mention Air France, assuming you are connecting in Paris then that is where you will go through immigration and enter the Schengen area, not Barcelona.

You will not go through formal immigration again after that. (If you were ending your journey in Barcelona you could walk straight out of the terminal).

If your onward flight was from the same terminal (it is not in your proposal) then you could do a simple gate-to-gate connection with many other airlines (but not all) - as you would have been security checked as part of connecting in Paris so could litterally walk up the jet bridge and change gates.
(Again assuming you are connecting, and that the air france refernce is not just a code share on direct flight from USA into BCN).

However, even if Ryanair were in the same terminal (again they are not, they are in the other one), you would still have a problem with doing a direct gate-to-gate connection onto Ryanair, because Ryanair are one of the small number of airlines who require (by company policy) non-EU/EEA passport holders to present themselves at their landside ticket desk (i.e. before security) to get their boarding passes stamped after the company has checked your non-EU/EEA travel documents.
It is not possible to do it airside, you would be denied boarding.
So, even if Ryanair had been in the same terminal, you would need to go landside to get your boarding pass stamped, then back through security.

Only a small number of airlines have this policy, but check.

I think it's just AF code but rather on DL476 JFK arriving BCN at 10:10am, so immigration happens at BCN.

Well if that is the case (it's an Air France codeshare on Delta), why mention AF at all in the question rather than Delta?!?
The codeshare is irrelevant.

Guess unless the OP comes back we will never know.