There are few combinations related to guns, and not many can be remembered now.
Mercenary assault rifle+electric chair (wheelchair+big battery) A car can be combined into a machine gun wheelchair.
Sonic gun+sound = large sonic gun, clearing artifact. Power is only for zombies, but for people.
The new combination of top secret files is electroacupuncture+sonic gun to form a stun gun, and then jewelry+flashlight to form a lightsaber. Then it is composed of stun guns and lightsabers. I forgot my name. But this is the most powerful gun in the game. Hit a zombie and enjoy the explosion.