I think it may be a combination of 2 things common to new shooters.
Firstly I think you probably overdid going for the first time, using that much ammunition. Its not "normal" to stand with your arms extended for more than a few minutes at a time. Try putting the gun down, or at least resting it on the front of the bench & relaxing for a couple of minutes before each magazine.
Secondly I've sen a lot of ladies shoot for the first time & they tend to lean backwards, unlike men who seem to lean forwards. I don't know why this happens but it is very common & gender-related. Again this is an unnatural pose & puts a lot of strain on the muscles of the back, shoulders, belly & torso in general.
Icy Hot works wonders.
Allan Quatermain: “Automatic rifles. Who in God's name has automatic rifles”?
Elderly Hunter: “That's dashed unsporting. Probably Belgium.”
Last edited by wogpotter; March 7, 2013 at 12:00 PM.