I was browsing the internet, and I found this page on yahoo answers, which says that the bayonet for the Garand can be unlocked by:
- Reaching rank 50
- Scoring 5000 kills with the Garand
http://answers.yahoo.com/question/index?qid=20100327100133AAKsFPd
I'm not sure if this is true, but it sure would be nice, if not a bit time-consuming.