To amplify what James K said...as most people aren't familiar with a machinist's level, the thickness of a piece of paper will throw the bubble completely off the scale. See this: http://www.starrett.com/metrology/me...hinists-levels
The cheapest route here is to take it to a gunsmith, and yeah, it may cost a few bucks for him to do it. But his cost to do it will be much cheaper than his cost to fix what you screwed up by not having the proper tools. This assumes that it IS repairable.