I am a noob to the AR world. Last night I decided to strip and clean all my regular shooters including handguns and ARs. First I disassemble the bolt on my 9mm AR and line up everything in front of me. Bolt, firing pin, firing pin spring, retaining pin etc. After I have everything cleaned and back together, I take apart the bolt-carrier on my 5.56 AR. Bolt, piston, firing pin, and dang, I must have dropped the firing pin spring...... So I start crawling around on the floor, looking for this little spring. After a while, I give up and decide 'what the heck, I'll just order one'. Started to look, of course there is no such part. The regular AR with the locking bolt uses a floating pin. The 9mm bolt is the exception, not the rule. If you look on places like AR15.com, and M4carbine.net there are some epic tales of internet knuckleheadedness revolving around that question.