I think when shooting off your arm muscle the gun has to be held close to parallel to your shoulder to shoulder line - shooting across your body. It may be a decent position for smallbore offhand work. But shooting a big recoiling rifle you want to make sure you are shooting the rifle perpendicular or close to straight away from your chest. The reason is your arm under big recoil is going to let the breech area make contact with your head. While the perpendicular motion is going to help move your head back before the breech area or particularly a scope can carve you some auxillary eye brows. I think you can prove this for your self by trying it with a modern rifle like a ,300 Mag - wear a helmet with face shield for the shot from your arm.