The C-Clamp grip and its effectiveness has been the subject of debate for about 10 years now.
However, I have never once seen a HAMAS member holding a weapon in the C-Clamp style.
Every single image I can find on Getty has Palestinians holding their rifle the traditional way - underhand by the handguard or by the magazine with their off hand. As of 2015, it seems like Palestinians forces were still
Palestinian from 2007. Hand on handguard.
Palestinian from 2015. Hand on rifle magazine
Palestinian Militants Using C-Clamp Weapon Grip