Question

What is the total momentum of the bullet and the gun before firing?

Answer 1

total momentum of gun and bullet before firing is zero...

Answer 2

We can derive the answer using the following method:

p = mv (p -> momentum, m -> mass, v -> velocity)
Here, m = x
but, v = 0 (body is at rest).

Hence, 
p = 0x
p = 0

Therefore, the linear momentum before firing is zero.