You'll have to obtain the proper muscle sets for you to shoot offhand reliably.
Find a sorta heavy gun --- and do gun mount to the shoulder repetitions --- 75 times a night. Accept the wobble of the rifle as it does circle eights around the target. In time...the wobble radius will get smaller. Try to use triangular skeletal bone support while shooting offhand.
Check out David Tubbs offhand shooting technique. Relax and breathe properly. If you do not feel right about the trigger press...do the preliminary shot process all over again.
For the prone position, which still has tips that might help you shoot offhand: