My back pain came from shooting 50's, here's the back story.
Our group started back in 2000, met on Biggerhammer when it was the place to go for everything .50's. Started out with about 8 of us if I remember correctly, I think there might be a pic of it still on EDM Firearms site. I usually go to the desert to shoot but figured that I would hit the range to meet some new shooters that were into.50's also. Most of us had just bought our .50's so it was cool to meet up with everyone and shoot our new toys.
Fast forward to about 2004 and we are all out in the desert at the spot I've been going to for years. Myself and one of the other guys were out at 1000 yards putting up targets so that everyone could dial in their guns for 1000 yards. It just happened to be really windy that day so we had to staple the targets to the 8 foot by 8 foot stand that was made for putting up the targets. So there I am standing on the bedside of the truck when the wind hit the standard so hard it blew the plywood off the frame of the stand. To say that I know what a fly feels like when hit by a fly swatter is an understatement!!! When the board hit me I got one step back into the truck bed and landed on my lower back on the other side of the truck bed and then flipped out onto the dirt. As I'm laying there in agonizing pain I thought for sure my backwas broken. So there I am trying not to lose my mind thinking that I'm going to be crippled for the rest of my life when the other guy comes running around his truck to see if I'm okay. I said I think so and start to check myself out while laying there, after getting up with his help I was done for the day/weekend.
When I got home the look on my wife's face said it all, I had a bruise the size of a basketball that went from just above my ass to about 1/3 the way up my back. She said I should go to the hospital but I said no even though I wanted to say yes. My back hasn't been the same since even though I did some physical therapy to try to get back in shape. On the plus side, MrBen that posts here has the whole thing caught on tape! Other then that my back doesn't hurt at all when shooting the .50 or any other firearms. It mostly hurts when I stand to long or sit in some crappy chair for awhile.