It happened to me about 4 years ago when I was 34 and I had some health problems which prevented me from shooting and hunting like normal. As my health has improved I started shooting the rifles which I used to love to shoot. OUCH!!! My 7mm Remington Magnum, 35 Whelen, 270 Winchester, and 308 Winchester all hit me more than I could stand. Even my light weight 20 gauge put a hurting on me. How could this be? I used to shoot these guns without pain, or so I thought. It is amazing what the body will endure with pain and what we think doesn't hurt. I hear people say you have to work up to the rounds with more recoil. My question now is "Why?" Now I have deer rifles in 250 Savage, 257 Roberts, 6.5x55, 7mm-08, 7x57, and the lowly 30-30. I just recently bought a 7.62x39 to try. I wish I had switched to these rounds earlier.
