But worry not! A bullet fired from 12 million light years away would probably be ablated into nothing by the interstellar dust.
Stop and think about the fact that starlight can travel that distance. Therefore the accumulated dust along that path is not enough to block light. That little dust won't stop a bullet either.
I can see starlight from the bottom of a pool, but the water in the pool will still stop a bullet.
(I am not a scientist, I could be entirely wrong, but it seems like the collective dust of 12 million light years would surely ground down a bullet, while being intermittent enough to let starlight pass through in aggregate.)
The collective dust of 12 million light years of intergalactic space does not seem likely to grind down a bullet very much.
That said, the whole calculation is flawed because first you have to get out of our galaxy, which is much higher density. Though that also wouldn't stop a bullet. But then you have our atmosphere, and that most definitely would destroy a bullet that tried to pass through it!
But the moral remains. Space is empty. Really empty. Unimaginably so.