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It sounds like you didn't
It sounds like you didn't realize you must zero your scope. You can't just put it on the rifle and start nailing the bull.
The link doesn't work for me. However, go to any auto parts store or Wal-Mart in the hardware section and ask for "blue loctite." It's like asking for duct tape. They'll know what you need. Blue loctite helps keep screws from backing out, but you can still remove the screws if you need to. Red loctite is semi-permanent.
I would google for some directions on zeroing a rifle scope. Let me know if you have any more questions.
Also, .308 isn't the end-all caliber out there. There's a multitude of great calibers. I just like calibers that are cheap to shoot, so I can shoot them more often at the range (9mm, 5.56mm, 7.62mm).