FlyerTalk Evangelist
Join Date: Aug 2011
Location: Barcelona, London, on a plane
Programs: BA Silver, TK E+, AA PP, Hyatt Globalist, Marriott LT Plat, Hilton Diamond
Posts: 13,053
Puente Aereo at MAD has its own security. It is a bit hard to find, I suspect precisely because anybody can use it if they want. There is rarely a long queue. As always, it's a question of safety margin + lounge time versus just arriving as late as possible. The gates to BCN are usually in the middle of T4, so not a long walk (assuming you figure out how to get down to the departure level without going all of the way to the trains for T4S)
At BCN, you do NOT need to repeat security, however the arrival / departure gates for the Madrid flights are way down at A1 / A2. It will be a long walk to the central part of the terminal, through the shopping area and up the escalators to passport control (near McDonalds). This assumes that you have your onward boarding pass already, as you are very unlikely to find somebody airside to check you in...
Whatever you do, do NOT blindly follow the people deplaning from the MAD-BCN flight. Many will take the Puente Aereo exit to go directly landside.