The bubble could be a deflector shield.
There is a scholarly article on the issue of what happens to matter (and light) which is encountered by a moving bubble. The popular press seized on the last paragraph where the author notes that particles overtaken by the bubble are swept along, accumulating energy and then are release when the bubble stops with an energetic radiant burst.
The rest of the article I really struggled with. I tried to see if there was a popularization, or perhaps a talk given by the author that explains the graphs better. No luck so far.
The Alcubierre Warp Drive: On the Matter of Matter
So you get my take. The author breaks it down for bubbles moving faster and slower than the speed of light.
For particles with initial positive velocity lower than the critical
velocity, there are two possibilities, either the velocity is above
that of the bubble and hence the particle will catch up to the bubble
from behind or the velocity is below that of the bubble and thus the
particle will in- teract with the front of the bubble first as the
bubble catches up to the particle. In the first case, the parti- cle
is ejected from the bubble back out the rear with a reduced but still
positive velocity below the ship veloc- ity. For particles with
initial velocity closer to the critical velocity, the path follows
that of the critical velocity par- ticles for longer, thus getting
closer to the ship, before diverging and being ejected from the
bubble. Similarly in the second case, the particle is ejected out the
front of the bubble with an increased velocity larger than the ship
Summary: particles and radiation definitely enter the bubble and they definitely can enter the normal space near the ship. They can hit the ship. An energetic particle (bullet?) hitting the front of a fast moving bubble can hit the ship - not with some weird superluminal velocity but a velocity related to that which is initially had.
A thing I struggle with: particles catching up with a bubble from behind are ejected back out the way they came in, SLOWER than they came in. A particle entering the front in ejected back out the front, FASTER than how it came in - here is your deflector shield.
This implies to me that the "moving" bubble would be spraying captured particles out the front the entire time it was moving. The huge spray of particles on coming to the destination is one case of this - all the accumulated particles coming in the front and destined to spray back out the front are released at once.
You want your deflector shield to sit in front of you. This would be a bubble with velocity 0. I do not understand the math well enough to figure out the path of a particle (bullet) entering a bubble at rest.
I am insecure in my interpretation of this paper. I really was hoping for a lay version but no luck. But at least here is some science by someone who does understand; maybe other readers will make more sense of it than I did. Links welcome!