I strongly caution against behind the neck barbell presses for a few reasons:
1. Many people do not have the ROM to bring the bar behind the neck without incuring some spinal flexion
2. And perhaps most important, the nature of a behind the neck presses place the shoulder joint at high risk for impingement of the RC muscles. The shoulder abduction + external rotation is a perfect recipe to impinge the supraspinus tendon and biceps tendon, resulting in tendonitis and eventually leading to weakening and even tears. And when that happens, how are you going to make your middle delts larger?
Answer, perform dumbbell presses at full ROM. Keep your elbows pointing in front of you at the bottom of the ROM, and let them gradually move out as the ROM is finished. Finish with your forearms equal to your ears (many cannot fully flex the shoulder when seated upright..a sign of a tight posterior shoulder capsule). Keep your shoulder blades squeezed together and down the entire movement (yes, this will reduce the amount of weight you can use).
Br