As you probably already know there is a proposed ban on common AR15 M855 ammo. A website has been made available, that will let you send a fax to the ATF stating your objection.
It only takes about 30 seconds to navigate to the website and send your fax. There is currently a back log of faxes waiting to be sent out as stated in the comment posted on their site. The website states that there have already been over 2,500 faxes sent.
Update as of 02/20/15, 16:00: It appears that we have exceeded the BATF’s ability to receive faxes. As of this update, we have approximately 2050 faxes still awaiting delivery. They appear able to receive approximately 1 fax per minute. At this rate, we have at least a 34 hour backlog. Our fax servers will retry as long as required.
The website is available by clicking the image above or following this link.