It seems that the main factor is behavior/personality.
If the dinosaur tries to be cautious he can take a lot of shots, and fear may or may not affect Walter much on the shooting part. And it's not like the lab is huge to have a large margin of error.
And because of the speed, Walter can use the lab to protect himself.
For example.
Walter could go through this place and use it as a kind of protection, right? I think it would be complicated to move the dinosaur, considering the size of the creature in proportion to the amount of space in the laboratory.