I sent 20 letters and got 5 farms to shoot on.
My top 'Letter' tip is not to ask for shooting permission but rather to only ask for a meeting in which to discuss the possibility of shooting permission. Sounds like a small difference but you are asking for a much smaller initial commitment which once granted is more likely to be followed by a second positive commitment. Standard sales psychology really.
AA Prosport.177, HW77.177, HW97K.177, HW98.177, Theoben Evolution.177, Theoben SLR98.177, AA S410.177, Alros Trailsman.22