If you really want to test it out chances are that a free game won't be able to push it to the lmits. Some games that are good (or ok), and free include:
America's Army - Probably the best free game ever - Wait for the new special forces patch.
Return to Castle Wolfenstein - If you like the one from a few years ago, here is a new free online version. - Probably be better to download that one
here .
If you looking for a free shooter that isnt online that is good, your chances are very slim.