He won't. He might lose a few, but he'll pick up a few, too.
There aren't many Americans who walk around pre-occupied with the assault weapons ban, other than a few here.
(By the way, I'd like to note that in my conservative circles at school just last week, several right-leaning students surprised me when they said, during some good war news, that they thought GWB probably wouldn't be re-elected. I, usually the activist Rightie among them, was shocked, and discussed how unrealistic that outcome was. Now, if he was already having the mushy supporters quavering last week, and he's losing some of us radical Constitutionalists and some of the single-issue gun folks, and the fact that he has increased the budget by a huge rate, and that he has passed CFR and the gun ban, and the fact the the Dems will cheat like CRAZY in 2004, you have to start thinking that there's real trouble afoot.)
Also, you seem to think that this will only affect the votes of those focused on this particular issue. But, this sort of thing sends a broader message to his core conservatives; namely: get lost.
If Congress renews this abomination, and he signs it into law, mark my words: he will lose, because conservatives will stay home. Also, the GOP will likely lose control of the Senate.
Mushy moderates will split their vote, because they dislike single-party control of government.