Question-Scope Mount

[Mohawked_Drums]

I just recently bought an 03A3 (30-06 not .308) at a local gun show... it had a scope mount and a scope with it... it was mounted on the left side of the rifile where the lever for the bolt realease is... someone had pulled the pin out, taken off the lever, and mounted a scope right there.. well i went and shot maybe 8 shots through it and the rifile was shooting all over a 15 x 10 dirt mound.. the pin that was holding the scope there had came out... any ideas of why it did.. or how to make it stay...