Hopefully someone can give me some pointers on this. I was zeroing my 308 and the wife's 243 yesterday for deer season and when I got down behind my rifle I got a cramp in my rib and lower back every time I settled in behind it. Had my bipod up to the 2nd from top notch (9-13) I'm running an mpa chassis adjusted for me. This happened one other time when I was shooting from a bench when I would reach for the paralax knob but no other time. Anyone else ever have this happen to them?