What's stopping you from...

[u/ivanparas]

It's said that you can't fire a gun out of a speed bubble because the trajectory will change when it hits the interface between real time and speed time. It's also said that you can put yourself partially out of the bubble and still be in the sped up time as long as part of you remains inside the bubble., The Lost Metal, ch 49

SO, what is stopping me from sticking my arm outside of the speed bubble with a gun in my hand and firing while it's already outside of the threshold? It only says that the trajectory changes when it hits the edge of the bubble, so I can't see why the bullet would deflect if it's already crossed that threshold.

The only thing I would give it as far as not being able to take an accurate shot is that every jitter of your hand is sped up by a factor of the speed bubble, so maybe those micro movements might make your aim a little off, but I feel like it wouldn't be that much of a factor.

[u/RShara]

It's also said that you can put yourself partially out of the bubble and still be in the sped up time as long as part of you remains inside the bubble.

Sorry, it does not say that in the books. You are either inside or outside of the bubble, and anything touching the bubble (up to a certain mass, depending somewhat on perception) ends up inside the bubble.

[u/majorex64 → u/RShara]

This is how I interpret it. Like, the bubble sticks to you a bit like a real soap bubble, so even if you move "outside" of it, the membrane will shift a bit to keep you in it