Momoka's skill is possibly hereditary. Kazue Sonokawa (her mother) possibly has a natural affinity with weapons just as Momoka does. She's possibly seen a bit of action when she was younger too and has the lessons of experience as an advantage.
Another contributing factor is that Kazue is an adult. Adults in Japan are allowed weild more powerful weapons than juveniles' airsoft guns. Adolescents can only use guns which are limited to a kinetic energy of 0.135 Joules. However, adults can use guns with 1 Joule - which could be why her bullets seemingly shot through the floor and also gave her an advantage over Momoka
(It was probably just the mangaka's decision though)