Air
The Air Arms guns are very popular. You will enjoy the gun.
You do not mention how you are going to get air into the gun. What method have you chosen?
In addition.....if you have not already purchased the S400....
Look at the Benjamin Marauder. Nicely made PCP. Less than half the price of the S400, more velocity, available in three calibers, can be easily adapted to CO2 if you wish.
I own the pistol version and it has proven accurate, reliable and powerful (.22)
Pete