Please forgive me for being new and jumping in here, but the other folks have diagnosed the problems pretty well. The only other point that might be added is that there is a tremendous difference in the dimensions among the manufacturers of .22 rimfire ammunition. I know that sounds a little strange, but if you look at the folks who are really into precision .22 shooting you'll find that they even use a guage to check each round as they take it out of the box because there is so much difference in the case size even within a single manufacturer's box that it can have an impact on accuracy. So, if you find that you get like strikes with one brand/type of .22 rimfire, try another--after you have thoroughly cleaned the bolt, extractor, firing pin as others have suggested. Best of luck and let us know what you discover.