Step 1: The force required to keep the machine gun in position is related to the rate of change of momentum. The rate of change of momentum is the force exerted on the gun due to the bullets being fired.
Step 2: The momentum of each bullet is given by \( p = mv \), where \( m = 50 \, \text{g} = 0.05 \, \text{kg} \) and \( v = 400 \, \text{m/s} \). The rate of change of momentum is:
\[
F = \text{rate of change of momentum} = \dfrac{\Delta p}{\Delta t} = \left( 0.05 \times 400 \right) \times \dfrac{30}{60} = 15 \, \text{N}.
\]
Final Answer:
\[
\boxed{15 \, \text{N}}
\]