So if the amendment passed, why didn't the assualt weapons ban end up getting extended? Presidential veto?
If you mean the 2004 vote... because even though Bush stated earlier on that he supported a ban on "assault weapons", he gave the measure the old nudge-nudge, wink-wink and made certain that it didn't get any support from the Republican controled Congress.
In fact, the NRA held off any endorsement of Bush pending the outcome of any AWB legislation.
At the time, the NRA was pushing hard for the "Protection of Lawful Commerce in Firearms Act". The charge was led by Senator Larry "Toe Tapping" Craig" (R- Idaho).
The antis had been trying to get several AWB bills to the floor, but to no avail.
The only opportunity they had was to introduce it as an amendment to the Commerce act (along with several other "poison pill" amendments).
As the commerce bill was belong debated, there were back room deals going on that allowed Feinstein, Schumer, Kennedy, etc, to bring these amendments to the floor.
The NRA's thinking was that if they passed in it the Senate, they could get them removed in the House version, and bring the bill to a committee decision with the result being the commerce act passing with no AWB.
That didn't happen though.
As the roll call was taking place live and on the Internet, many pro-gun Senators offices and the NRA's office, were being flooded to kill the bill altogether rather than extend the AWB (and close the "gun show loophole", require trigger locks).
Fortunately for us... they actually listened for a change.