[alt: The tumbleweed then tried to roll off into the sunset, but due to the Old West's placement north of the subtropical ridge, the prevailing winds were in the wrong direction.]
I was basically at a total loss for this one, not able to come up with anything beyond "this is so boring i can barely stand it." Luckily, the old adage that "every single commenter on xkcdsucks is smarter than Carl" proved true once more, as an Anonymous poster noted that this comic is much improved with the simple reversal of panels 3 and 4.
It's true - as it stand, panels 1 and 2 set up that there is a duel (that's fine), but 3 sets up the idea of a tumbleweed with a gun. What can possibly follow that that would be funny and subvert expectations, we wonder. Then we read panel 4: the tumbleweed shoots them.
The problem is that there's nothing in panel 4 that our minds didn't immediately assume based on panel 3. Far better to have them shot first, and have us wonder who shot them (or did they shoot each other?) and then have the reveal be the wacky twist - it was the tumbleweed all along! the tumbleweed! that mother fucker.
Anyway, that simple reversal isn't quite enough to make the comic right - for example, I think it would be better to have the tumbleweed appear far right in panel 1, and then a little more to the left in panel 2, and delete the awkward "TUMBLE!" sound effect in panel 2. You'd also want to get rid of that weird little vibrating effect on either side of the tumbleweed in what would be the new panel 3, as it draws attention to the surprise too early. Still, these are small changes, and would lead to a far stronger comic.
It's still not a great comic - I mean, it boils down to "hey guys! what if the TUMBLEWEED did it!" - but it's somewhat solid. Much better than the utter bore we have.