Callaway Big Bertha X-12 Pro Series. Callaway Big Bertha X-12s were the natural progression from the original ground-braking Big Berthas. The low centre of gravity in these Callaway Big Bertha X-12 irons help to get the ball airborne more easily with the proper trajectory and spin from almost any lie. The unique channel running around the inside of the cavity allows Callaway's designers to create a thinner top profile - and optimise the weigthing for each individual iron. Callaway Big Bertha X-12s feature a low centre of gravity to help get the ball airborne - and a deep cavity, with plenty of weight around the outside to help straighten off-centre shots. They are just a bit prettier than the originals; less angular, a little smoother to ensure less drag out of the rough - and a more forgiving hitting area. This Callaway Big Bertha X-12 Pro Series set has less offset than the normal model - allowing better golfers to work the ball more easily