Don't worry about it, just have theses responses in waiting.
If he asks why you need a HMR over an FAC Air Rifle, You say that you need the extra stopping power at range and it is more versatile than an FAC Air Rifle.
Also, If asked why not use a shotgun, You say that the noise and ammunition costs of a shotgun do not make for efficient pest control. Shotgun Cartridge=35p, HMR round=20p
I wouldn't think he will ask you why, he shouldn't any how if you have sound reasons.
The above answers are sound and watertight, they are above and beyond the normal reasons you need to give.